TL;DR - Yasuo is a champ that gets extra value from items, and also exists in a state where he massively benefits from the mechanically skilled, but outside of them he flounders between good to average based on who he faces, but he can be too frustrating to play against since the vast majority are the people even average Yasuos can excel against.
I think Yasuo suffers from a very familiar situation where he is statistically underpowered but kept in this state because a good number of people would find outrage if he were to get any meaningful buffs. It's a symptom of the fact that while playing him is one of the most stellar experiences League has to offer, fighting against him can become especially infuriating considering he has the potential to block meaningful spells with Wind Wall on top of being able to poke (and potentially engage on you) from a surprisingly safe distance.
Is he Zed? No, but he can be obnoxious at times for certain people who favor the champions Yasuo excels against or can mitigate to some degree; yet his poke and his mobility is conditional and therefore telegraphed easily, which is why he struggles at higher elos where people naturally can deal with him far more easily.
At lower elos, people just lack the means to properly punish him for doing what he is meant to do, and they don't punish him properly for certain behaviors (trying to poke him when he tries to last-hit to pop his shield, not baiting out the Wind Wall before starting a fight on him, not reacting properly after noting when he has his tornado ready, etc.), and often follow the worst behaviors that he wants (staying near your own minions when he tries to dash through them and/or throw his tornado through, throwing everything at him when he has Wind Wall off cooldown and/or minions to dash through, trying to dive him when he has his ultimate and a tornado ready, etc.)
He sort of exists in a place where he excels against people who just aren't good and is only average against people who aren't bad, and anything to try to put him in a better spot will just prompt backlash from the majority of players, who statistically make up the lower elos which have problems with Yasuo.
That isn't to say Yasuo doesn't have legitimate reasons to be kept underpowered, he is mechanically complex and can excel even in high elos with people who have excellent mechanical mastery with champions like Lee Sin, Syndra, Azir, Zed, etc; this combined with the situational power of his kit can make him quite frustrating to deal with, because he can invalidate anyone with important long-range skillshots (a lot of ADC ults like Ashe's arrow or Jinx's Rocket for example), and his mobility that in the right situation means he can be as impossible to catch as he is to escape.
Ultimately, though, I think the biggest problem with him is that he is secretly a value champion, he gets a lot out of items more than most champions because he has things in his kit that increase his overall value when getting stats, he has a refleshable shield that is generated from moving which increases the value of movement speed on himself by reducing the need for health in his build, he has the double crit passive which massively increases the value of crit on himself and devalues the need to focus crit in his build, and he has the bonus armor penetration on his ultimate when used that greatly increases the value of the damage he deals.
is sprinting towards me in ult? Just dash through the creep wave with an E that still does damage, rinse and repeat.