Specifically, how game tries to treat "surprised" mechanic.
If you try to shoot someone from stealth and out of combat - they will not be considered surprised.
Basically, that means that auro-crit mechanic of assassin subclass pretty much never works.
If you want to make assassins viable, and you don't know how to properly implement "surprised" status in combat. Then just change assassin's ability: "as long as assassin have advantage on attack rolls, in very first round of combat all his attacks automatically a critical hits"
