Yi is not overpowered. He definitely should be removed/reworked though.
He is fine if - and only if - the enemy team composition has explicitly prepared for handling it.
But here is the thing. Why do we have to enforce certain hardcore cc team compositions to be able to deal with a single enemy champion potentionally going wild? Why do champions exist that can take over an entire game based on a lead?
How is that top goes 0/4 at ten minutes to Master Yi, and there is absolutely no chance to stop him, unless we have a point click stun and superior focus?
I can even understand that there should be champions who can takeover games, but if they capable doing just that, they should require very high skill to do so. Riot did realize this btw, and 'newer' champs are more complicated to takeover games with. See
,
or even
- or almost any newer assassins. Most of these guys are really hard to play or have built-in mechanisms that limits their capability (e.g. zed relies heavily on his ult), thus you simply won't be consistent destroying teams with them. That is exactly the way they should be.
Yi is different. He requires absolutely no skill whatsoever. You only have to know when to click on the enemy, and occasionally use q. That's it! (You have to do that with every single champion in the game btw.). He is too consistent taking over games when he gets ahead, that is why he has to be removed/reworked.
Any champion who pulls an amazing win-rate constantly and against which the only answer is hard cc and
should be looked at very seriously.