yuumi is pretty disappointing and she suffers from the same thing reworked soraka (rip 2013 soraka, never forget) suffers from and thats being relatively terrible on her own. While potential damage output and offmeta builds like full ap/ap caster vary throughout the subclass, soraka was pretty alone when it came to her relative uselessness by herself until yuumi came along.
Both suffer from the problem of practically having a single damage skill with mediocre output (soraka's e's cooldown relegates it to cc as the primary use). In sorakas case she also lacks an effective self heal outside of her ult and in yuumis case she has some of the most abysmal health/resistances in the game. In both cases this leads to near complete reliance on teammates for any semblance of efficacy which would be tolerable if they excelled at helping their allies compared to others in their subclass but they do not.
Yuumi really missed the mark, adding the attach mechanic was definitely something many people had been wanting for a long time but it really doesnt make up for the loss of self control or aa's. Her w could easily be point and click and her passive could easily be transferred in a more conventional way. This means her w is basically a minor adaptive damage buff which by itself is not worth a slot. Imo her passive is nice but theres not really a way to lower the long cooldown so its efficacy is pretty poor considering the fact that going in for an AA often collects some enemy fire which nearly or entirely nullifies the capacity. If it wasnt for her sona-like ult she'd really have little to no useful depth to her kit.
The community definitely deserves more than just one enchanter after 4 years since nami was released and I really hope riot looks towards characters like orianna and kayle who both made good enchanters (before 7.16 for ori and a rework for kayle where they were ruined) but were also competent in their own right.