They're nerfing it not because it's OP or because Master Yi is powerful, but rather because they've had plenty of complaints about it and it's not fun to play against. For better or for worse, Riot usually listens to the community. Sometimes that means perfectly balanced things get nerfed because enough people QQ over it. Another example is Blitzcrank's nerfs. Annoying? Yeah. Overpowered? Of course not, and yet his mana pool is now nonexistent early. Coupled with the increase to his Q's cost, he can only throw out 2-3 grabs before running out of mana.