I'm not too sure if they fixed Assassin's Ambush. Last time I tried it wouldn't trigger on the first attack and would only apply after starting combat (I know it used to work in older versions of the game so I don't know if this interaction was unintended or not)
Assassin is still playable and you can do some silly things with Durge to get the Deathstalker Mantle and utilize the invisibility after killing something to reset encounters and constantly re-engage, as even without that initial guaranteed crit you can still get a free attack in then have your action refreshed by Assassin's Alacrity so you still get a full round of attacks against the now surprised enemy.
Though, Gloom Stalker and Thief tend to be better Assassins sadly (Thief because you can abuse the Cloak of Cunning Brume with the double bonus action, allowing you to disengage to make a fog cloud and then stealth because no-one can see you again allowing for encounter reset cheese. You can also fog on someone and take them out when you have the Eversight Ring)
