My issue with assassins is only that they can solo turret dive so effectively so early. Being 70%+ hp and being under turret while lets say the assassin is at 40%- hp should make you feel pretty safe but that's such a mistake if they are level 6.
Their kit is made to make them and their squishy selves be able to go deep into the back line kill someone and kill high priority targets and get out to a reasonable level of success.
The issue with this is it allows them to do very stupid things like not care you have a turret when they are at level 6 particularly with leblanc, zed, and akali. This could be solved with some very simple changes:
Zed... cannot blink back to R shadow if enemy champion is targeted while under turret. W cannot be blinked to if zed placed it while being targeted by enemy turrets.
Leblanc, cannot blink back if she uses it to enter enemy towers range.
Akali, can use w to drop tower agro but cannot re-enter w if she attacks allies under enemy towers after w is placed.
People want to call them high risk high reward champs but I don't really see what risks they take turret diving so safely relative to every other kind of champion. Its more like high risk to play against if you are one of the majority of champions that lack a good way to deal with them without having to go a high defense build ruining so much of the value you were hoping to have later on just cause of what the enemy picked alone.