Yuumi Needs to Be Reworked
Okay been gone for a bit and only been playing normals to get back into shape and thought Yuumi looked fun, First game enemy got her, we won, easily, enemy team's Yuumi was useless. Second game we got her, we lost, mostly because it was a 4v5. but with Yuumi it felt like a 3v5. I wasn't even sure if I wanted to play her at that point but I had to for the sake of firsthand experience, and it sucked. Sure it didn't help I got teemo as my laner and he was the most passive person constantly keeping me out of range to use my Q for poke or running away when I fired it, but later I latched on the jungler primarily. Did pretty damn well with him, but my numbers were very low, a 32 minute game going AP support having to build mana regen because her mana consumption is terrible and only went 15k damage.
Her passive goes against her entire kit, and I guess is supposed to support juggling in and out of W, because she can't auto in W so she has to go out and this is essentially just a xerath passive but she also is super squishy and going out once just to hit an auto to regain mana and a minimal shield just makes her a target and a very squishy one. A new passive is a bit tricky because currently she needs all the mana regain she can because her mana consumption is just ridiculous, but it should be more in line with her kit of being attached and supporting someone else. I'd say the current one needs to be scrapped all together and her mana consumption needs to be fixed and just give her a passive more in line with her kit.
Her Q relies on it becoming empowered to slow and do decent damage, which requires a 1 second of flight time, but if she is attached to a melee champion engaged in combat, her Q is pretty bad, just have it apply slow no matter what or be empowered all the time, it only does decent damage, its weak compared to most support pokes and the enemy team can just hide behind minions.
W is decent love the passive love the ability to become untargetable, but as soon as her partner dies which is somewhat likely because being untargetable means they can't possibly split focus meaning they are free to attack and harass her partner and if she goes out to block it generally she takes pretty hefty damage on account her squishiness. Still think it's pretty good in its current form tho.
Her E relies on the target either her or her partner if she is attached being lower on health to do even slightly decent healing, and it only has 10% scaling, the lowest scaling heal ability in the game, lower than Sona's which also shields allies and heals 2 people. At the very least it needs to be able to heal herself and her partner at same time if she is attached, with no reduced numbers, her healing is dismal, at minimal they need to make it on par with sona's healing scaling minus the shields.
Her ult is actually quite fine in my opinion, the maneuverability and firing style of a Lucian ult with the area and kind of effect of a sona ult but only rooting not stunning while doing more damage than sona if you hit all 7 waves on same target.
TL/DR: Passive needs to be scrapped and mana consumption fixed. Her Q needs to slow all the time at the very least if not be empowered just from using it in W. W is fine. E is the worst healing ability on a support in game, it only heals one target based on missing health and even the missing health doesn't scale up much while it only has 10% ap scaling, so it needs to have increased ap scaling imo and also be able to heal both herself and partner in W. Ult is fine.