You know, Im perfectly fine with Yasuos kit by itself.. but
why is he allowed to be so tanky? Passive shield that prevents him from being bursted 1v1 unless you are really fed, windwall that can block key spells from many champs like Lux Q, Fizz ult or even Talon W to deny him 3 passive stacks. Then there is PDs stupid dmg reduction, defensive boots and other items like Frozen Mallet, Triforce, GA, Maw, Deaths Dance and life steal in general since he reaches 100% crit and pretty high damage with 2 items anyway so he doesnt really need to build that much more dmg.
When he came out this wasnt really a problem since most Yasuos just built
into even more damage so they were actually killable ( https://i.gyazo.com/669ae00f318d9e63c45776bdd67e9442.png for example is the first season 5 build guide I found).
Its fine that he can melt tanks with free armor pen on ult and its fine that he can cut down your squishies with like 4-5 autos. But what isnt fine is that he can facetank a full Talon combo because he hit him once with PD and bought NInja tabi without even blocking anything with W.
Edit: Yes Yasuo has counters and yes he has counterplay, but that doesnt mean he cant be overpowered ever. Azir has counters too, like every champ in this game, yet he was overpowered af after release + bugfixes/buffs.

.
