Hm, what could possibly go wrong.
Yasuo cooldown: Q - 3.7-1.3 rank 1-5 E 0.5-0.1 sec rank 1-5 ult 79?/27 sec rank 3 - Manaless, wind wall, shield, double crit, 50% bonus armor pen for 15 seconds
The problem is that Sion needs time to level. Yasuo cannot be punished if he makes a mistake with Q/E because they are right back up when he casts them. Sion makes a mistake he's out a chunk of mana and a decent cooldown. You farm with Sion, Yasuo will charge in and smack you around to almost no health. If you get a gank, he'll roam mid or bot and come back stronger.
Bottom line: I've seen this idiotic champion go 0-5 in lane and end up 25-7 or higher by the end of the game - lack of cooldown, ridiculous mobility and damage, and Conqueror even existing makes this a joke. How do you outplay someone who can reset his abilities as if its constantly URF mode?
Play safe, he gets stronger. Play dumb, he gets stronger quicker.
This is one reason I was on Team Riven last year! 