Versus City posted a translation of an interview Arcadia Magazine published where they asked Daigo, Mago and Tokido their thoughts on Super Street Fighter 4 Arcade Edition. Here's a short snippet, hit the link to read everything.
Well, let’s get down to it. The strongest character in the game is considered to be Yun, but how is he, really?
Tokido: I’ve tried out Yun, and his defense is really bad.
Mago: His defense?
Daigo: That’s really his only weak point. So if you’re an offensive minded player and can keep the pressure on him, you should not have a lot of problems against him. But if you can’t surpass his offensive capabilities it’s over.
This is part 1 of the three part series, more will be posted in the future.
Most people are saying Yun is better than Yang. In terms of versatility, I can't say I disagree. I think he has more options in general. Up kicks come out faster than yang's roll, and while Mantis slash is incredible for pressure, Yun has both his shoulder and lunge punch. I'd still say Yang is pretty high tier though.
Yun can't FADC his Up Kicks... so while he has invincible wakeup options, he can't do them safely.
Daigo didn't get a 89% win rate through luck though.