- He gains too many free stats, and gets to become tankier with his buildpath.
For example, doubled crit chance; Most champions need to get three crit items, and even then they can only gain 80% crit chance at best, by doing that. Yasuo, on other hand, gains 100% crit chance just by getting 2 items. He already gets 100% crit chance with just a 
and a 
early! Yes, Yasuo's crits have their damage reduced, but that doesn't matter that much, as he gets to crit way more often when compared to other champions.
His ult also gives him free armor penetration. Although it only applies to bonus armor, and has been recently nerfed to apply only to his crits, this still makes a lot more difficult for tanks to fight Yasuo, or to survive against him. Itemizing HP may help, but not once Yasuo builds lifesteal.
All of those free stats also allow Yasuo to have a way more flexible build path, since for example, with the armor penetration on his ult, Yasuo doesn't have to build a 
, unlike other carries. He also doesn't have to build more than 2 crit items, and thus can focus on getting more raw AD and lifesteal from items like 
or 
or 
, all which make him a lot more tankier and harder to fight. Basically, Yasuo is allowed to invest on survivability because his kit gives him a lot of natural damage.
Let's not even talk about his passive, which essentially gives to Yasuo more effective HP.
Oh, and because his Q cooldown is tied to attack speed and his E has a instant cooldown (only being limited by on-target cooldown) Yasuo doesn't have to itemize for cooldown reduction either. His Q cooldown get short by building attack speed which is a stat that he benefits a lot from. Only his ult benefits from CDR and it already has a very short cooldown.
- He is completely resourceless
He simply doesn't have to manage any resource. And unlike most other resourceless champions, he doesn't have to build cooldown reduction (as I explained above, his Q's cooldown scales with attack speed, and his ult has a short cooldown already). This makes him a lot harder to fight early game because he can win lane just because you don't have the resources to fight him. He also has a lot more freedom to stay in lane because he only has to back when he is very low on HP. Champions like Pantheon, Malphite, Fizz, Malzahar, or Jax, could have a much better time against Yasuo in lane, but unfortunately, they are mana hungry, so Yasuo can win against them simply because he can outsustain and outlast them, even if they are technically considered counters to him (That's why I consider Yasuo to have no actual counters, as he can win every single lane possible in one way or another).
There is a reason why most resourceless champions have strong early games. They are usually balanced because they tend to fall off late game when other champions can start to better manage their resources or they have better ways to deal with the resourceless champions. But in Yasuo's case, he doesn't fall off. He actually scales incredibly well into mid/late game.
- He is way more mobile than he should be
Most people say Yasuo has infinite dashes, which is false because his dashes are limited by a static cooldown early, and are limited by the amount of nearby enemy targets. This doesn't make him any less mobile, he still has outstanding mobility early because he can abuse your minions to get in and get out whenever he wants. He can use this to trade with you and give you no chance to fight back, especially if you are melee. For example, if he has his tornado ready, he can dash, get to you, knockup you, auto, and get out, giving you no chance to damage him at all. Even if you damage him, you still have to deal with his shield, so he is incredibly hard to fight early. That's why Yasuo, despite being good against rangeds, is also a nightmare to fight against even as a melee.
- He completely negates the range advantage of many champions
Usually, being a ranged should give you the advantage against a melee. Even if you have the tools to gap the close against a ranged, you may still be able to be kited if you mess up. For example, if Zed doesn't use his shadows properly, a ranged will still kite him. This is not the case with Yasuo. He gets to close the gap on you with his E, which hardly has a cooldown, and can abuse his tornado to CC you and have a chance to close the gap. He also usually builds Frozen Mallet, an item that makes him even harder to kite, because he slows you and even if you hit him from distance, the extra HP makes him have a better chance to survive.
- Unlike other melee carries, Yasuo has utility
Usually, champions like Fiora or Tryndamere are not allowed to have hard CC because otherwise they would be broken (Fiora has a stun on her W, but it is extremely situtional). This is not case with Yasuo. He has the second best CC in the game on his Q (which is a suspension). Even though it is a skillshot, it is very spammable, costs no resource, and has a low cooldown, in addition to a decent range. Yasuo can actually make it targeted with the help of his E. Yasuo can zone you early with the premise of knocking you up (which is even more dangerous after 6 because he has his ult).
He also has his windwall, which can block even many ultimates, and makes fighting against him as a ranged even harder because he is blocking your auto-attacks.
- Yasuo is weak when behind
If Yasuo doesn't manage to snowball, he is weak and can't do much. He has utility but not that much to the point that he is useful even when behind, for example, like a Lulu mid would. He also gets to do little damage and be too squishy if he can't get his items. He is very feast and famine, not that much when compared to champions like old Kat, or Riven, but still, he is feast or famine.
- Yasuo abuses items too well
Phantom Dancer: He abuses the damage reduction passive like no tomorrow. He already has decent waveclear (making Statikk Shiv less of a need on him) and is already difficult to outduel. With this item it is almost impossible to outduel him as any champion. There is a reason why the majority of Yasuos rush this item. It even gives him a lot of stats, and makes it even hard to escape from him or kite him.
Statikk Shiv: Although most Yasuos build Phantom Dancer, he can build this if he wants to have even better waveclear. It also gives him burst, and because of his passive which gives him double crit chance, Yasuo gets even more chances to crit with this item's passive.
- Yasuo is completely balanced around his skillcap
Like Azir and Kalista, it is almost impossible to make Yasuo balanced for lower tiers or players which are less skilled with him, without completely breaking up him on higher tiers, or at the hands of people which have already mastered him.
This also means that a very good Yasuo is a god and an even below average Yasuo is garbage.
- Yasuo is too team composition reliant
Unless you have knockups (other than Yasuo's himself) on his team, Yasuo find himself too reliant on hitting his Q to engage. However, if he manages to get champs like Malphite or Orianna as his partners, he is obnoxious, because not only you have to watch out for the champions themselves, but you also have to watch out for Yasuo.
- Yasuo is indeed a case of failed design
Yasuo was designed to be a melee carry at bot lane, replacing the ADC. That's why he has been given so many anti-ranged tools, in addition to making him resourceless. However, those tools have been abused to make Yasuo an obnoxious solo laner.
