Master yi is overpowered, why? Maybe not overpowered, but outdated.
Master yi is an old champion along with Garen, warwick, ashe, etc. Guess what happened. They all got reworked, and especially warwick completely changed its identity because of his "toxic, unhealthy and abusive" gameplay after a moment of severe nerf (aka the biggest direct nerf known in league of legends history, literally).
Master yi is an outdated champion. He does not fit in the modern gameplay.
They thought warwick was OP when ashe could 1v1 him all day and shit on him. There was nothing warwick could do against ashe. Warwick running away? Keep chasing him and auto him with that slow. Warwick following you? Just keep running away until he turns back, which is when you'll start to auto him again. Warwick ults you? Qss (or ability block item) and ult him right back and position yourself. As long as you're above 50% health, what's warwick gonna do? Absolutely nothing.
There were a bunch of braindead kids playing with insane mobility champions who were stupid enough to duel warwick for a minute, mid to late game, and complains about how warwick heals too much when warwick ends up outdueling them. Jesus Christ. Lee sin couldn't outduel warwick 30 min into the game. Nerf warwick. Heal op gg.
Warwick's dueling power wasn't even the op part of him. And they blamed everything on the feral flare. If anything warwick's most useful kit was his e, which gives movespeed boost as well as the vision of low health enemy champs. If you see how the modern game is played, people spend crap ton of time chasing down and looking for low health enemy champs only to get burst down or juked. Well that wouldn't' have worked when warwick had the e. It might have hurt the gameplay where people who play assassins who only knows how to abuse that kind of psychological stuff to get to high elo, but it didn't hurt the gameplay overall, imo.
So, imo, warwick was op in low elo, solo queue where people didn't know how to ward, have no clue of enemy presence on the map and lacks coordination that they chase down low health enemy champs for eternity.
But with massive w nerf, the immobile warwick became a complete garbage. His dueling power was so incredibly bad that he had trouble during 1v1 with amumu. (if an immobile champ has no dueling power, what makes of him?)His already slow clear speed was massively slowed down even further. Made him unplayable. Turned him into a troll pick. I'm not even sure what was riot's thinking in doing that? Xin and olaf has gap closer or ranged attack. Warwick has none but his ult, which was also nerfed and either way had 1min> cd. Literally, the enemy champ could auto warwick once, let him pop his w, walk away for a couple of seconds, and comeback and dash to him and destroy the shit out of him. Who in the world thought that was the balance?