He's a hyper carry. If anything he gets stronger as the game continues. I still see many Yasuo's sporting the tank-hybrid frozen mallet build and still oneshotting carries with ult, when steps were specifically taken to remove this type of gameplay. All he has to do is land one single knockup in a fight, or just wait for someone else to knock up and then he presses a button and automatically wins the teamfight. And if he is building the tank-hybrid build, you're punished for trying to focus him in a teamfight.
Mechanically, you can't even interrupt him mid-ult, so even if you're playing a support who can peel, it doesn't matter. He will ignore all CC during his ult, and still do the full damage, while your entire team is CC'd by his point and click bullshit on zero cooldown for zero resource. That is, even if he is bad and doesn't windwall any of your CC anyway. Even if you outplay him and do everything you're supposed to, it doesn't matter.
When Zed ults a carry, you have a huge window of opportunity to stun/knock him back/kill him before he can do damage to trigger the mark. Yasuo, nope. Doesn't matter what you do, as soon as the animation starts, your entire team takes full damage and CC no matter what.
His hyper safety and low cds with zero resources make him a beast at CSing. It feels like no matter how hard you shut him down or outplay him, he ALWAYS, without fail, is able to come back into the game and carry if it continues past 30 minutes. Having him windwall just to shut down your CSing is extremely frustrating as well. I have never seen any Yasuo at my bracket have less than 250 cs by 30 minutes, when most other laners have ~170ish at the most. Even if he's 1/10, 250 cs is nearly a full build.
I normally ban Vi and Lee Sin, but sometimes I ban him instead. He is just such an irritating champion, and the deluge of complaints and fixes on him kind of solidifies this as an issue.
Normally I never advocate trashing a champion, and in almost every case, there's a way to outplay/counterpick/overcome most supposedly oppressive champions.
But this asshole is just so irritating I wouldn't be the least bit upset if he was Evelynn'd. If he was reasonable to deal with, certainly there would be guides/posts on how to deal with him. But no, all it is, is just complaints about how nobody likes playing against him. In every single elo. He even sports a higher banrate than LeBlanc. That should be proof enough how much people hate playing against him.