Yuumi needs work, interesting character! but needs work.
So, stat wise, Yuumi is a piece of paper that gets cut in half by the slightest pressure. You know those plastic wraps or paper with the notches poked in it to make tearing easier? That's yuumi. Low hp, abyssmally low stats.
But then she just doesn't seem to have a great stat base for her kids. Here are some suggested adjustments. Two changes to passive I will suggest: Passive Allow passive to proc on Q while mounted. (Oh BUT NO, SHE HAS NO REASON TO UNMOUNT???) Bruh, yuumi's already will be avoiding unmounting, because the benefits from autoing a enemy champion/trading via autos isn't worth it. OR---- Allow her passive to proc on large minions, monsters, and champions. That way she can auto siege minion, get shield and mana, then scurry back inside her carry.
Q Give it a small amount of splash. So that Yuumi can help in some god forsaken form to wave clear or survive on her own if she's forced too.
W Great, keep it, it's FINE.
E This is cool too, low healing but overall with her ability to just be a living potion inside of someone, it's actually great.
R Up base damage and increase root duration.
Her passive is by far the most limiting factor about her.