I'm prepared to get downvoted into oblivion for this, but I actually think he's in a good spot. He's not super powerful early game in lane, but he gets a massive power spike at level 6 when he has his ult, and he can do some serious damage in exchanges. His grab is very useful, and his e is a really good "let's get out of this situation without dying" move. A lot of his power budget is used on his ult, which is phenominal at all stages of the game. He's never tanky and can die very easily if you're not careful, but even with low income and items, he's always got a way to catch up if you can land good ults for kills.
I don't think he's OP, I don't think he's undertuned, I think his win rates are accurate of the fact that he's different (most supports powerspike at the beginning and fall off as the game goes on, he powerspikes a bit differently and plays differently) and I think that if riot leaves him alone, he will gradually become an accepted, balanced champ.
He's not a true assassin; he can't find people and 1v1 kill them unless they're not smart or he's stupid ahead. He cant instagib very well. He's really a support assassin; he plays all the main components of an assassin, but in a supplimentary way.
I don't think he needs any balance changes. I think he's in a good spot.