The only reason to actually pick him over Vayne, Jinx, Tristana, or Kog'Maw, who are all late game hypercarry ADCs that blow everyone up, is because Twitch's teamfighting can POTENTIALLY be more rewarding for the most part.
I think giving him 0 > 2 > 4 more damage on his ultimate won't solve anything or lowering his mana costs from his Ambush. Both his Q and Ult are completely fine.
It's his passive and W that are both extremely useless in most cases. Late game at max rank, with full stacks on a target, with the full duration, they take a mediocre 216 true damage. That won't even leave a dent in tanks with over 3,000 health and 100 or so armor to mitigate his auto attacks. And his W has relatively no uses. It has a 25% slow for most of the game which won't do much, it doesn't do damage and only applies 2 stacks of his passive, so aside from ambushing people or wave clear It's extremely useless, oh and not to mention Twitch's E is just a watered down Kalista rend.