Helldritch
Hero
Our rogue considered Assassin and decided against it - it simply won't happen often enough to justify the subclass. When the whole group wants in on the action, having one party member scout ahead and control when the fight starts isn't fun.
If the feature wasn't so very restricted and curtailed it might have worked, but it is. Besides, to gain it, you have to lose out on Thief things. I'd much rather give the base Rogue some oomph.
The assassination feature is as restrictive as you want it to be. By the same token, it can be as unrestrictive as you want it to be.
Imagine this: A lone unnoticed assassin, is getting into a room from one of the windows. It is the sleeping quarters of the guards. In the room there are two sentries 30 feet appart doing a surveillance round and a dozen other guards are sleeping in their beds. Can the lone assassin can assassinate the whole bunch of them as long as he do it quitely. Get to the first guard kill him quitely, take an arrow/bolt/dagger, shoot, kill the second sentry and then butcher the rest of the sleeping guards. It all depends on what you decide to consider combat. Is there combat when no one notice the dead guard? Or as soon as someone dies, combat is on? I would allow that scene without a second tought.
Second situation.
An assassin shoots with an arrow an unsuspecting guard. He is a wood elf so he uses his ability to hide in the forest. Now everyone know that an arrow came from the forest. But where exactly did it come from? Would you allow a second, third, fourth etc... assassination attempts until the assassin's position is correctly guessed? And if the assassin only attack once every three rounds so that he can manoeuver the poor search parties and starts to assassinate them at his leisure? Would you consider each assassinations as a mini combat or would you consider it the same combat? They are aware, for sure, but there is no clashing of the swords. Should combat only truly start when the assassin is caught red handed? 10 guards (whatever they are) can make a good perception check with advantage to notice where that arrow was shot from. As long as the hide roll bonus action of the assassin is higher than their perception roll, then do you consider the assassin to be out of combat or is he in combat from the very first arrow?
Both examples assume that the target dies from the assassination attempt. This is the kind of thing we see all the time in fiction and both of these situations have happened in my game. With the result that the assassin got his way as he wanted. There were other time where he failed and a normal combat ensued. Would you allow a human assassin to do the same in a heavy/dense foliage where he could get cover? What about fog? Or in darkness where the assassin could position again and again? These are the kind of things that are part of the call of the GM. Do I make the assassin too strong by applying this ruling? And what about the thief that get such a high initiative roll that he can back stab two times before the enemy can react?
The rogue can be quite deadly when he can do his job (and he is allowed to). No fighter could do any of the scenari above. It would just be, kill the first guard and try to position himself so that he would not get flanked (if you use that rule as I do). On the second scenario, he would simply flee. Does that make it a bit dull for the other player to witness? Not at my table. They would simply congratulate the player and be thankful for the saved resources that his daring allowed.
I also saw that scenario: Your dagger cuts deep into the guards back. But he does not fall. He turns around and you see that he was already dead. His red gleaming eyes glare down at you, a fiendish grin forms into a lipless grin as the undead unsheats a black ebony sword. Your soul will be mine, little one...