Yuumi's weakness is they aren't a full champ in terms of what they do. They are basically anywhere between .25 and .5 of a champ. Their biggest asset is the fact they can follow champs in on routes most supports cannot take, and being ganked by 1.5 champs is still better than being ganked by 1.
Play yuumi a bit. I mained them for a long time. It was fun at first. You are a buff not a full champ. It did take more skill originally but they kinda hammered down the skill ceiling and raised the skill floor a bit. But it's still just meh.
The q barely slows, the heal is nothing unless you get unholy grail. The adaptive force you give to the other player is cool, and the ulti is great, but you can only nerf a champ so more before a rework happens.
They got 5 big nerfs in a row. Chill out. They are bad now. They got a 43% win rate.