There's a particularly extremely frustrating bit to Yasuo that really keeps Vel'koz from reliably fighting back.
Vel'koz E has an INCREDIBLY large window of time and space to get vaporized by Yasuo's windwall. You can understand the time part because it doesn't matter how long the projectile's been out, as long as the windwall comes out in time it'll get blocked. But space? Where does that factor in?
Try this incident on for size: A Yasuo dives Vel'koz under tower. He dashes into me, so he ends up behind me. Sad thing is, he windwalled right afterwards so the wall spawns about 200 units behind me. Yasuo realizes he screwed up by towerdiving, so he starts walking back to lane. Note I haven't moved an inch because I'm getting ready to blow my load on him. EWQ, because Yasuo has nothing to dash to, so I'm confident in landing this standard combo on him.
I use it in the direction he's walking towards, so all these spells are being thrown AWAY from the windwall, right? It's 200 units behind me, so it's not even touching my hitbox, right? The E marker appears right in Yasuo's path, with the W and Q coming out FLAWLESSLY, but...
..the E marker never exploded. It got BLOCKED. BY A WINDWALL THAT WASN'T EVEN IN THE WAY.
tl;dr It is incredibly frustrating to fight against a Yasuo as Vel'koz when windwall is so difficult to play around. The difference in the amount of skill with which a Vel'koz player needs in order to defeat a Yasuo is rather wide, all due to the reliability of windwall's interactions with Vel'koz's abilities.