One of the most infuriating and frustrating champs in the game to play against and sometimes it feels unfair when I'm playing her. The fact she spends the majority of the game untargetable is insane. She provides whoever she's attached to an insane amount of damage in the later game.
But I think what annoys me most about her is that damn q. It's borderline undodgeable "skillshot" and having to lane against it is a nightmare. It honestly feels like yuumi has no weak point throughout the game. Her late game scaling is nuts with the stats she provides for her attached ally. And in lane phase, the constant q harass that you can't even dodge is impossible to lane against.
The only thing I could think of is to make her lane phase much worse. Make her q cost more mana when attached, make it slower, I don't know. I love playing yuumi. She's one of my favorite supports in the game to play. But she shouldn't be this damn strong.