Twitch is a better designed champion. He isn't op busted in any realm, but he performs well because his kit has things like stealth, which is stronger in solo queue due to lack of coordination.
Yi, on the other hand, can't be slowed while ulting, runs super fast while ulting, scales fucking hard, and can dodge incoming damage with Q. To make him better for higher elos, he'd legit break low elo in two on practically any body part he wanted. When I make posts about champs like him, or Garen, or Teemo, or Kayle, or whoever, people down vote spam me and "argue", but then they catch feelings when they realize the cold truth: Older champ kits just need modernization if they want to be real contenders in the game without pissing off 50% of the community like Lulu does.