Ult gives full Flow instantly
Sure, but it also locks you up for the ult animation, during which any random damage from any member of the team you didn't hit immediately pops your shield and leaves you unable to cast windwall. This would make ulting in a teamfight a pretty guaranteed way to ensure that you won't be casting windwall for the next few seconds.
it really should just cost half Flow, not all of it.
I personally think the entire idea of using flow as a resource is inherantly flawed. Having a resource that serves primarily as a way for random enemy damage to disable combat skills is going to be incredibly frustrating to play with. It's unnecessarily annoying relative to the amount of power it takes away, and the counterplay it introduces is too straightforward and easy to utilize for his opponents. On top of that, the expectation it places on Yasuo is ridiculous. "Take no damage, or lose access to your skill". Taking 1 point of damage and losing access to what should ideally be a reactive spell is silly. That's not something he can control, and expecting players to control it is absolutely unreasonable.
Flow really isn't set up to be a good resource, because good resources don't deplete to 0 instantly based on something as uncontrollable taking a single point of damage. It would be atrociously unfun to actually use, and it's not like this is our only option for balancing and/or redesigning Yasuo. We're not lacking for options here. We don't need to jump on something as bad as this.
I think it may be cathartic for people to see Yasuo be given a resource given the general resentment the boards have for manaless champions, but I don't think it's actually good design. Regardless of how much I may dislike him, I don't see any good reason to select a balancing option whose primary effect would be annoying his players, particularly when we have tons of other options for balancing him.