I was just playing a game and i saw ashe firing her ult towards yasuo who was chasing her, at the same moment, yasuo shot his Q (the tornado thing) towards her, Ashe ult did hit him, but while in the middle of the stun he went HOYOYOYOOOOOOOOO and nothing was left from ashe but her ashes
I do not know if this was a part of his ultimate or not but this is surely not fair, no other champ can use an ability while under a stun, so why should yasuo ?
Yasuo can't ult when stunned. I cannot even count the number of times I have had perfectly played knock ups on multiple champions; only to get my ult interrupted by some speeding Hecarim or blitzcrank grab. He probably already activated the ult, and got hit by the stun mid ult.
can ult while stunned, but 
being able to do it is a little bit more ridiculous.
What I'm guessing is that Yasuo used the ult a moment right before he got hit by the arrow; if the arrow was shot at close range then the stun would have likely worn off before Yasuo finished completing his ult animation. It's also possible that he had a 
His R animation probably just started before the stun. He doesn't instantly teleport.
When you add in lag time it's very possible.
as an avid veigar player that has played vs yasuo countless times, i can conclude that he can indeed ult while stunned.
certainlyT champion, what did you expect?
Yasuo can't ult while stunned. I wish so much that he could, given that he's my main, but he just can't. Either he managed to start the ult before he got hit, wind walled it, or it was a bug. But he definitely cant ult while Hard CCed.
Adding to this why does Yasuo get a ranged auto after his Q or able to pull off a near instant triple auto on top of his Q + E. As if he isn't already annoying enough. As for the issue being discussed he probably started the ult as he got stunned and so it completed and then he was stunned after if duration was still left over, or the Zhongya bug issue occurring with stun. I think the CC should end his ult in this case, tbh. It is already pretty powerful and CC's everyone in the effect for extended duration meanwhile you have ults like Kat/Nunu/Galio/etc...
This is probably a bug
Yasuo has lee syndrome, the people are knocked up, he MUST ult he just cant resist it
He cast Q3 before he got stunned.
League is different from games like Guild Wars where you're able to interrupt enemy abilities.
Yasuo is broken, I have seen him multiple times being able to ult off of me ulting him with orianna, as if me pulling him in with the ult made him able to ult me and ignore the cc my ult does, actually a broken and annoying champ almost worse than ekko.
I have never been able to ulty while stunned and believe me a mash that R, dunno what happened there.
Yasuo cannot ult when stunned.
+1 for HOYOYOYOOOOOOOOO
Yasuo can't activate his ult while stunned but if he is mid ult he won't be interrupted by it. It works similar to Lux' ult, if you stun her while she is charging the laser she will fire it. It can be considered to be like a ~1second cast time instead of a ~1second channel. Cast times are only interrupted by death (+ supression and knockups?) while channels get interrupted by hard CC.
there's that, and then there's him ulting while reviving from GA/zilean ulti but at least they fixed the bug where he could ult while actually dead and waiting to respawn
Sure they can. Ez and lux can both finish their ults so long as they began casting Beforehand. Which is what this Yasuo did.
ThE only way to stop it is the best cc: death.
If he was stunned like a second before the tornado hits and still able to R, it's a bug.
If you saw him R RIGHT when the arrow exploded, then he DID hit R before being stunned, hence, worked as intended, some abilities are un-interruptable once cast.
He wouldn't be balanced if he couldn't
