I would NOT call Fiora's ult utility. I would also call it lousy, but that's less because it's a bad ultimate and more because it's a stupid ultimate for her character. "I am a duelist, that's why I have an ult that gives an AoE heal. You know... because duelists totally heal people". Fiora's ultimate is thematically unfitting and was just included to force her to have a use in teamfights and correct the "melee carries suck" problem Riot was having at the time.
But this is beside the point as I never mentioned Fiora. I mentioned Yasuo... who is built from the ground up in a way that is as frustrating as possible. Just because higher skill players are good at ruining him doesn't make him well-designed. Winrates don't equal good design either. Statistics can be made to say anything, really... and even when they're given earnestly they can be very misleading.
Imagine there was an item in the game. Let's call it the Gambler's Coin. It costs 2000 gold, gives NO stats and has only one use... which happens immediately if you purchase it and leave spawn. When it's bought, the game randomly destroys one of the two nexus', ending the game immediately. Now, this item would probably have very close to a 50% winrate, since that is literally what it does... half the time it destroys your nexus, half the time it destroys the enemy's. But ANY idiot can tell you that this is a very poorly designed item. It's stupid and unfun. It will only be bought by teams who are about to lose, in the hopes of securing a win they shouldn't have earned... And by trolls a lot, probably.
Now, Yasuo is not THAT bad. But his kit is full of those kind of cockblocking or unfun mechanics. Did you poke him? Well his passive shield diminished how much that poke mattered. Try to ult him? Wind Wall. He's ahead of you so you need to play safe? Good luck laning when you can't step near your minions without getting dashed into. His kit puts ALL the rewards on him, and all the agency on him. Yasuo's kit means nearly every lane or matchup he's a part of is decided by Yasuo. Even if his high difficulty means that Yasuo often decides this to his detriment because he is easy to mess up, the fact that he is firmly holding the ball and you can mostly just hope that he fumbles it is BAD DESIGN. And this would not be so bad if he wasn't snowbally, or if he fell off over time without making plays... but he is brutally strong with mid-late game items... so you are basically put into a position where you kinda HAVE to make plays and get ahead of a champion who is entirely built to cockblock you. NOT GOOD DESIGN.
You're looking at numbers, winrates and whatnot and saying "oh man, if he's not overpowered than he must be well-designed". No. It doesn't matter if high elo players are good at exploiting his weaknesses when he tries to playmake, or if his difficulty makes him hard to use or any of that. He is poorly designed by his very nature, and no amount of numbers fixes will change how much players hate his guts because playing against him is frustrating in the extreme. THAT is why he is toxic. Because he is a toxic, agitating influence on the game. All nerfing him will do is make his winrate plummet so that the people on his team hate him too. Because they'll know he's bad and unlikely to do well because of his low power and high difficulty. He needs a kit change to make him less rage-inducing, not a buff OR a nerf. I don't want him nerfed. I want him tweaked so that his mere presence in a game doesn't bring on seething waves of hatred from people. He needs a kit that FEELS more fair. That is less frustrating to play against. That doesn't feel like playing cops and robbers vs the kid who insists he has an "everything-proof vest" that makes him immune to your bullets. It doesn't matter if he loses eventually if the entire game he's annoying as sin and puts everyone in a lousy mood.