If you played a Thief and didn't use Fast Hands, I'd question your imagination. You have caltrops, ball bearings, animals traps, acid, alchemist's fire, holy water... all from the PHB and all great to use as a Bonus Action. You can devote entire builds to using this feature in every round of every combat.
The picking of locks at 30 feet generally isn't all that useful. Most traps go off when you open a door, not when you pick its lock so there's often VERY little reason to need to pick a lock from 30 feet away. At best, it's a cute roleplaying thing.Also, if we're going to compare PHB Rogue subclasses... why not compare Assassin to Arcane Trickster? At level 3, while the Assassin gets an ability they can use maybe a handful of times in any campaign, Arcane Trickster gets three cantrips and access to the Wizard spell list, one of the best in the game, while ALSO getting the ability to pick locks and disarm traps from a distance of 30 feet.
How many D&D campaigns involve picking locks + disarming traps and would greatly benefit from an awesomely safe distance for both? I'd say all of them.
If your party rolls well on Stealth, if you roll well on Initiative AND if you don't miss... oh, also, if your party is fully on board with changing their entire encounter approach so that you can bend the campaign to use your class ability, as well. And assuming your DM allows the type of scouting/sneaking ahead that would be required and assuming the encounter would even allow the party to be hidden (a Big Bad, a target you'd MOST want to get an opening-round Crit on, might be fully aware of the party for their planned monologue, as an example).
And don't forget, if you scout ahead in the dark using darkvision, your perception checks are made at disadvantage....
There's a lot of debate as to what counts as a "Use an Object" action. LOTS of debate. Most people believe that throwing Alchemist's Fire or Holy Water is an Attack Action, since it requires an attack roll. Most of the DMs in my area rule it that way, including me, so I wouldn't try to use those.
The rest of those things are also somewhat debatable if they are "use an object" or not. Either way, most of them have nearly no effect on the combat and just slow things down so I don't bother
The picking of locks at 30 feet generally isn't all that useful. Most traps go off when you open a door, not when you pick its lock so there's often VERY little reason to need to pick a lock from 30 feet away. At best, it's a cute roleplaying thing.
Yes, yes... spells are BARELY more helpful than the Assassin's Level 7 and 13 abilities to look and sound like other people. If only there weren't a Level 1 spell the Arcane Trickster can get that would allow them to Disguise their Self...Spells are handy, and yes, are likely more powerful than the other features...though not much more.
Yep, those are the conditions required. In my experience it means getting an assassination off once in every 10 combats or so if the Rogue tries. I have been nice to people before and allowed the group to make a group stealth check to sneak up on enemies. So if 50% of them succeed in stealth, they can get the surprise which I find is a little more fair and has stopped a lot of arguing over "Your stealth sucks! You need to stand 100 feet behind us at all times!"
However, even with RAW it isn't all that difficult to sneak up on enemies if you try. Even someone with +0 Stealth and Disadvantage still beats the average PP of 10 or 11 about 30% of the time. Rogues generally have a good Dex and tend to go before monsters a majority of the time.
So, if the entire party says "We're sneaking constantly", you should still see an assassination happen about 10% of the time.
The rest of the ability where they get advantage against things they go before in initiative happens almost every combat.
The only real use I've found for Fast Hands is with the Healer feat.
Of course you are aware of a threat, there's someone standing in front of you casting a fireball spell. That seems like a threat to me.Not at all. As soon as the player declares they're casting fireball, combat starts. Decide surprise (was anyone aware of a threat?) and roll initiative.
No, there's no in game rationale for why surprise ends during your first action. As I said above, it is a bug that is a side effect of removing the idea of "rounds" from the game.What doesn't happen is "oh they have time to react, roll initiative."
The entire point of surprise lasting only until your turn is that there is no time to reassess threats before your turn.
So, what happens if someone is hidden, they are completely unseen and unheard?If the Assassin goes to stab the player one of two things happen:
The player rolls higher on initiative, sees the glint of steel and has time to recover from being surprised.
The player rolls lower on initiative and is caught by surprise.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.