In less than 5 seconds, he was able to nuke me down while I was fed Riven. Over 1.5k damage with STEEL TEMPEST only. 500 damage from his ult and over 500 with just basic attacks. Only 4 seconds and 3k damage with Steel Tempest AND basic attacks. You can't run from him because of E. You dont dodge his Q which is spammable. His high attack speed with items lets him push towers extremely fast with auto attacks and reduces the Q cooldown.
His passive doubles his critical strike chance. He doesn't need a whole lot to get crits on most, if not all basic attacks and Q's.
you were riven and you lost to yasuo moreover you were a fed riven do only i think " wtf " riven eats yasuo
You are supposed to blow him up because you are an assassin. Don't get knocked up, don't waste abilities and your full combo burst kills him before you die to his consistent damage.
However, he will scale harder into lategame because he has two more multipliers in his damage than you (both have attack damage and armor penetration, Yasuo also scales with attack speed and critical strike, granted, Riven scales with cdr, but not in a single combo). His base damage is lower, but with items it scales harder. Yasuo also keeps dishing out the damage with his autoattacks long after you spent all your cooldowns on your combo. Don't let him hit you after your combo (all his mobility is based on either enemies or knock-ups, don't flee through your minion line just like you wouldn't run through bushes from a Rengar).
I've never seen a fed Yasuo lose
His high attack speed with items lets him push towers extremely fast with auto attacks and reduces the Q cooldown.
Former-- "push towers extremely fast"-- is quite false. Yasuo does not build a lot of turret-applicable on-hit damage; as he builds Critical Strike instead...
And the latter is obvious, because his Q cooldown is not affected by cooldown reduction.
Additionally, it is possible to run from a Yasuo-- just don't run towards your own creeps (which, I know, sounds paradoxical). You can dodge his Qs (though to be perfectly honest, this one is close enough to impossible as Yasuo gains AS).
Lastly, sometimes his basic attack damage is rolled into his Steel Tempest casts, like how Vayne's Tumble appears alone even if Vayne's performed basic attacks outside of Tumble.
Lastly, 1.5k from Steel Tempest in <5s is somewhat... irresponsible data. 1) How much armor did you have? 2) How much health did you have? 3) How much Armor Penetration did he have? 4) How much damage did you shield? 5) What items did Yasuo have?
Those are all important; and some of them can't be known at all...
His Q whirlwind hitbox is inaccurate in my opinion.
ok so most of us know that 
passive is double crit chance but 10% lower crit dmg, the problem is how this works with 
that makes this passive pointless since ie boosts crit dmg to 250% i think needs to be altered to give 50% more crit dmg (so if ur at 200% u get 250% if ur at 190% u get 240%) not just giving the full crit dmg, unless im misunderstanding how works in that it alters ur crit to 250% not adds on i think this would help some at least
