Master Yi is broken, not overpowered.
Basically what the title says. I've been around since 2010 or so and I've been on and off with the game, and I haven't quite played ranked for anything more than the promo 10 games.
Master Yi's kit is just broken, in the sense that it allows for either no counterplay or he will get shit on. As someone who got dominated by Yi's, who learned to counter them and who eventually even tried playing him, he's the most bipolar champion in the game, and it's just not healthy for him to exist in the game as he is right now.
In lower ELO, players aren't aware of how to counter Yi- by picking hard CC and applying extreme lane pressure early game. In higher ELO, just doing those two things is more than enough to make the Yi wanna rip his hair out.
Players playing against him in low ELO outright can't fight him properly, while higher ELO play has him get shit on. He'll either stomp the shit out of you or get stomped himself, and you can fully prevent him from doing so.
He lacks any consistency without items. There's no "middle-ground" match for him, there's no back-and-forth with the enemy jungler, he's either gonna work at 100% or 0%, and it just isn't a healthy kit. In my eight years of playing this game, I've yet to have a match with a Yi on either side that was close. Either he was fodder, or he was an unkillable god. Only having those two states clearly isn't doing anything for him. Besides, if Yi gets fed, he's just winmore by that point. He only works if you're ahead, but he doesn't contribute anything to getting ahead.
I think Yi could use a rework.