So I was playing some games with Yasuo on PBE. Overall, his ulti is great- he has CC, damage, and even if it is situational, it is very powerful and skill oriented. However, nothing makes you feel more disappointed than being interrupted in the middle of it. For example, you get a great 3-4 man knockup- not an easy feat. Then that pesky cho drops a knockup of his own on you in mid ulti. You end up dealing no damage from the ulti, being CCed (and probably dying because of it, even if you have half a brain and don't go 100% glass cannon), and the CC aspect of your ulti is either so short that you can't notice it not ending early, or the CC ends early. All that marvelous work, completely wasted. You feel like you had no effect on the fight at all beyond your initial Q.
So the obvious solution is to make him invulnerable to CC during his ulti (after that animation starts), or at least to set it so that it has full effect even if interrupted.