Not that I think Yi is OP, but to play devil's advocate: you can CC anybody... so saying that "you can CC Yi" isn't an argument as to whether or not he is balanced. Stun Camille, and she can't do 1000 true damage with her 2nd Q. Stun Graves and he can't lifesteal 500 health with one auto-attack. Stun Malzahar and he can't flash ===> ult your ADC. Stun Nautilus and he can't ult your ADC. Stun Vayne, and she can't 360-no-scope-mlg-swag-1v5-gosu-god-mode your team.
The only real exceptions are Olaf, who can be immune but only with his ult, Fiora is she predicts your CC and times riposte correctly.
Now, you can break the CC with Gangplank, Rengar or Alistar, but that's not the same as completely denying the CC at all.
But yeah, circling back, you can't use "X champion can be CC'd" as a point regarding their balance, because they can ALL be CC'd.