Twitch fell into the trash can (Not that he minds) when Riot gave him some serious nerfs post VU.
His Passive might as well not exist, Q is worthless as a escape mechanism if you get hit once because it takes 6 fucking seconds to go in stealth, Kalista's Rend alone is much better than half his kit (Passive stack + E).
He is still a monster late game, but literally every ADC bullies the fuck out of him and he is gank bait. It is laughable that people act like Vayne is such trash early game when Twitch exists.
What are his gimmicks? AoE late game? Jinx does that almost as well while being twice as safe and having an early game. Stack mechanic? Kalista laughs and kills Baron from 2k HP in a single spell.
Stealth? If he uses it to fight he cant escape. If he uses it to escape, he loses his attack speed steroid (And even then, it is incredibly hard to survive long enough to escape).
He has an interesting niche late game as an assassin/ADC, but Quinn has a similar ability with her Ult and he doesnt burst anywhere near as well as Graves or Lucian.