I've only played a few games of Yuumi, but after playing her and watching some gameplay, Yuumi just doesn't so much well. She can't poke, peel, heal, or shield as well as any other champion.
I think she has a lot of potential as this playmaker enchanter (Like Rakan), but she's just outclassed in every aspect of what she offers her team. Conceptually, she should thrive in team fights where she can help the Frontline engage, and also bounce back to heal the backline. But then in the middle of that she's supposed to auto for shields (which feels awkward).
Her E is 2 Bard heals Ws a higher cooldown.
Having to auto with short range to get a shield is hard when if anything touches you (especially in a team fight) you just pop.
The Q travels too slow, so unless you are attached, it probably won't hit.
Her ult is interesting, but so many times the person I am on mispositions it with a dash and the it's useless.
Q needs a damage buff (would be nice if it has splash damage/slow, E needs a way to recharge (like taric).
Would be cool if when you are attached, each ult wave goes in the direction of your mouse. Or you get 7 charges (similar to Ahri) and you can use them separately.
Maybe more attack range to help with passive? (Annie range? xD)