Jinx is fine, just gets a bit too much free stats from her minigun but she is very vulnerable to assassination and may be a bit overloaded but doesn't really feel cheap or toxic to play against by design.
Vayne shouldn't be able to do so well on her own, peeling off assassins AND killing them unassisted, while also having a great late game.
And before anyone brings up Tristana, yes she also had 2 disengage tools, but one of them had a high cooldown and the other was literally her ultimate, and her good self-peel was half the reason she got gutted reworked.
are much better designed hard carries. Twitch actually has an extremely weak early game, when he falls behind he stays behind, and has only one shot to position himself when he decides to teamfight or attempt an assassination, if something goes wrong he's generally screwed and can't tumble away or condemn an assassin. Kog is hands-down the most vulnerable ADC to assassination, and relies heavily on both his positioning and his own team to stay safe and get his damage out.