Let me start this by saying I'm a Master Yi player. I know him inside and out. Even with the small recent buffs to him, he's NOT op.
He can be dealt with in numerous ways. See, a real Yi main has something called "Risk Assessment". We wait out for the enemy team to use up all their stuns and major burst, then we go in and do what we do best.
BUT for those who dont have that skill, Yi gets hit and shut down hard.
Master Yi Pops Highlander, and runs head first into a team fight after the initiation, then all the enemy team hits him with all their CC, keeps him from doing anything then kill him. All his damage is gone.
Master Yi cannot make good use of Lethality. He already has it with his E - Wuju Style. Every single Yi I've seen who goes full Lethality always ends up feeding horribly because he doesn't have the fighting potential a Yi that runs full Attack Speed does.
In my mind you can only build him 1 of 2 ways. Full Attack Speed Assassin, or as a Fighter Tank. That's it.
If he goes FASA, then he's squishy as fuck and can be burst down and CC'd to death.
Fighter/Tank can't do as well as FASA thus not as useful in the fight. More Meant for Dueling.