reviving dead post because I find this issue especially interesting.
The real issue is in how the assassin subclass is worded, which is a total mess.
The exact ruling for surprise in 5e is counterintuitive and a bit weird, but once you understand how it works it actually makes sense.
The surprise mechanic is meant to define a tactical advantage so to reward strategic thinking (or punish the lack of it).
If you prepared, planned ahead and succeeded in ambushing your target, you basically get way ahead in the action economy, which is huge in 5e.
You strike out of nowhere, before anything is resolved initiative is rolled, all the creatures unable to detect you now have "the surprised status".
Then everyone acts per initiative order, as usual.
All the creatures with the surprise status, though, can only use their turn "to stop being surprised", which is an enormous advantage.
Do they come before you in the initiative order? Well, now they know you're there, but they spent their first turn doing nothing.
Did you get to go first? Even better, the creatures can't even take reactions until after their first turn.
The difference between the two scenarios is minimal. In both situations a succesful ambush means a lot of free attacks and a very evident advantage for the ambushers.
Using invisibility, stealth, or enchantment spells or whatever it's absolutely worth it, it almost always defines the outcome of the encounter.
I personally dislike this system, but I admit that even if it feels clunky it actually works as intended.
The problem is that the way the assassin is designed goes against the core rules of the game, it makes absolutely no sense. It's so bad it's offensive, because the assassin it's actually a great subclass, but it makes the worst possible use of the game mechanics. Seriously.
The whole point of the class is using planning and role playing to take down enemies instead of engaging in "normal combat". Every single feature the assassin has revolves around this concept.
If you are inventive, smart and plan together with your party, you can turn combat encounters in puzzles or in social encounters. This means that the assassination actually RESOLVES OUT OF COMBAT, because you spend most of the time preparing for it. This is obviously balanced by the assassin's complete lack of proper combat skills. Outside a narrow and very specific situation, the assassin is by far inferior to any other rogue subclass. He literally has 0 combat bonuses and 0 exploring or utility oriented skills.
This a super cool choice because IT FORCES YOU TO COMMIT to a role play heavy style, rewarding you for interacting with the game world and putting you at a steep disadvantage if you don't.
So you have to take everything in account: you stack bonuses, buffs and potions to rise your chances to hit as much as possible, you stack damaging buffs, poisons etc, to be sure to do maximum damage, you study a strategy to make sure to reach your target without alerting it.
You make sure you have advantage, you make sure you can cause surprise on your target.
you plan, gather and expend resources, build and optimize your character to do so.
You never have a 100% chance of success, but you can do a lot to rise your chances by as much as possible.
Because it's what the class is designed to do, because it's what you wanted to do, because it's super fun.
BUT THEN, THE WHOLE MECHANIC FALLS APART.
Because the assassin absolutely needs to activate its ability, or all the planning done means nothing.
If you have to strike while the target is still surprised, then you need to make sure you have a higher initiative than it, right? yes of course. TOO BAD, YOU CAN'T.
THERE ARE ALMOST NO WAYS TO INFLUENCE INITIATIVE ROLLS.
You just can't prepare for it. And if you fail, you're done.
The few that exist HAVE NOTHING TO DO WITH PLANNING, strategy or preparing. And almost all of them ARE NOT ACCESSIBLE TO THE ASSASSIN. The only thing you can do is take the alert feat. Period. If you don't, the class is unplayable, because there would be NO WAY to influence the outcome of the roll that decides if the assassination works. The whole class is based on roleplaying, planning, being sneaky and strategic, it's all about thinking in advance. But all the work you do is actually pointless, because in the end you just hope to get lucky and roll higher than your target, which could very well have dex modifiers not so different from yours.
I hope you see now why the wording of the assassin MAKES ABSOLUTELY ZERO SENSE. it's so dumb it's infuriating.
It's horrible design. And it's even more infuriating because the class is actually well made and balanced, but the wording makes it unplayable., unless your DM decides to change the surprise mechanics.
It would take so little like to fix it, like: "all your attacks toward creatures who have been surprised this round are critical hits" or a different variant with different implication "during the first round of combat all your strikes directed to creatures that haven't taken any actions yet are critical hits". with this wording your planning will always work as intended if you did it properly. You can still miss, you can still roll low, you're taking CALCULATED risks to get a great reward, which is very different than "you plan for hours then toss a coin and hope it wasn't all for nothing".
Or they could leave the core ability as it is and add another ability like the one the 3.5 assassin had: "if you study your target for at least 3 rounds while being no more than 30 ft from it and without the target being aware of you or considering you a threat you get advantage on the next initiative roll".
This way you can do something! You can influence the roll, if you plan thoroughly and find a way to decieve your target or prepare a trap. That's what assassins do both in games and real life!
Or they could even leave things as random as they are and add a greater reward.
"The assassin's critical strikes multiply the total number of dice by 3 instead of by 2". If the intent was to design a class that employs extremely risky strategies, then rise the stakes! This is even more flavorful because ithighlights the assassi's anatomical knowledge and proficiency in murder.
I hope i made my point clear...
The real issue is in how the assassin subclass is worded, which is a total mess.
The exact ruling for surprise in 5e is counterintuitive and a bit weird, but once you understand how it works it actually makes sense.
The surprise mechanic is meant to define a tactical advantage so to reward strategic thinking (or punish the lack of it).
If you prepared, planned ahead and succeeded in ambushing your target, you basically get way ahead in the action economy, which is huge in 5e.
You strike out of nowhere, before anything is resolved initiative is rolled, all the creatures unable to detect you now have "the surprised status".
Then everyone acts per initiative order, as usual.
All the creatures with the surprise status, though, can only use their turn "to stop being surprised", which is an enormous advantage.
Do they come before you in the initiative order? Well, now they know you're there, but they spent their first turn doing nothing.
Did you get to go first? Even better, the creatures can't even take reactions until after their first turn.
The difference between the two scenarios is minimal. In both situations a succesful ambush means a lot of free attacks and a very evident advantage for the ambushers.
Using invisibility, stealth, or enchantment spells or whatever it's absolutely worth it, it almost always defines the outcome of the encounter.
I personally dislike this system, but I admit that even if it feels clunky it actually works as intended.
The problem is that the way the assassin is designed goes against the core rules of the game, it makes absolutely no sense. It's so bad it's offensive, because the assassin it's actually a great subclass, but it makes the worst possible use of the game mechanics. Seriously.
The whole point of the class is using planning and role playing to take down enemies instead of engaging in "normal combat". Every single feature the assassin has revolves around this concept.
If you are inventive, smart and plan together with your party, you can turn combat encounters in puzzles or in social encounters. This means that the assassination actually RESOLVES OUT OF COMBAT, because you spend most of the time preparing for it. This is obviously balanced by the assassin's complete lack of proper combat skills. Outside a narrow and very specific situation, the assassin is by far inferior to any other rogue subclass. He literally has 0 combat bonuses and 0 exploring or utility oriented skills.
This a super cool choice because IT FORCES YOU TO COMMIT to a role play heavy style, rewarding you for interacting with the game world and putting you at a steep disadvantage if you don't.
So you have to take everything in account: you stack bonuses, buffs and potions to rise your chances to hit as much as possible, you stack damaging buffs, poisons etc, to be sure to do maximum damage, you study a strategy to make sure to reach your target without alerting it.
You make sure you have advantage, you make sure you can cause surprise on your target.
you plan, gather and expend resources, build and optimize your character to do so.
You never have a 100% chance of success, but you can do a lot to rise your chances by as much as possible.
Because it's what the class is designed to do, because it's what you wanted to do, because it's super fun.
BUT THEN, THE WHOLE MECHANIC FALLS APART.
Because the assassin absolutely needs to activate its ability, or all the planning done means nothing.
If you have to strike while the target is still surprised, then you need to make sure you have a higher initiative than it, right? yes of course. TOO BAD, YOU CAN'T.
THERE ARE ALMOST NO WAYS TO INFLUENCE INITIATIVE ROLLS.
You just can't prepare for it. And if you fail, you're done.
The few that exist HAVE NOTHING TO DO WITH PLANNING, strategy or preparing. And almost all of them ARE NOT ACCESSIBLE TO THE ASSASSIN. The only thing you can do is take the alert feat. Period. If you don't, the class is unplayable, because there would be NO WAY to influence the outcome of the roll that decides if the assassination works. The whole class is based on roleplaying, planning, being sneaky and strategic, it's all about thinking in advance. But all the work you do is actually pointless, because in the end you just hope to get lucky and roll higher than your target, which could very well have dex modifiers not so different from yours.
I hope you see now why the wording of the assassin MAKES ABSOLUTELY ZERO SENSE. it's so dumb it's infuriating.
It's horrible design. And it's even more infuriating because the class is actually well made and balanced, but the wording makes it unplayable., unless your DM decides to change the surprise mechanics.
It would take so little like to fix it, like: "all your attacks toward creatures who have been surprised this round are critical hits" or a different variant with different implication "during the first round of combat all your strikes directed to creatures that haven't taken any actions yet are critical hits". with this wording your planning will always work as intended if you did it properly. You can still miss, you can still roll low, you're taking CALCULATED risks to get a great reward, which is very different than "you plan for hours then toss a coin and hope it wasn't all for nothing".
Or they could leave the core ability as it is and add another ability like the one the 3.5 assassin had: "if you study your target for at least 3 rounds while being no more than 30 ft from it and without the target being aware of you or considering you a threat you get advantage on the next initiative roll".
This way you can do something! You can influence the roll, if you plan thoroughly and find a way to decieve your target or prepare a trap. That's what assassins do both in games and real life!
Or they could even leave things as random as they are and add a greater reward.
"The assassin's critical strikes multiply the total number of dice by 3 instead of by 2". If the intent was to design a class that employs extremely risky strategies, then rise the stakes! This is even more flavorful because ithighlights the assassi's anatomical knowledge and proficiency in murder.
I hope i made my point clear...