just me or do assassins have some..difficulty.

so upon saying this I have played assassins I think...2 times. one on blood moon and just did a rengar game. it was horrible upon me leaving the premade and getting a champ shard it was an assassin. though if there is anything lacking from that rengar game it was my judgment. all the games of have made it REALLY hard to do something that's somehow even squishier. and cant say I have done many,or really any champs the require 'mechanical skill' if that's what you call it, pancakes for all I know. when exactly is it ACTUALLY time to strike as an assassin cause I don't like almost getting oneshot by some lv 7 ADC