Yuumi so far...
As far as playing the champion goes, she's not bad and quite fun. But i'm not very sure yet if whether or not she should be buffed but that's too soon to speak. She's a fun champion really, but unfortunately i got all losses. People not knowing how to play safe and farm, damage is too damn high to heal off and shield, and of course, me messing because i'm not use to the cat and what to do at the last second reaction.
Anyhow, 1 thing i always have to keep in mind is that i should not lock on to my camera view (force of habit due to playing Sona) when i try to aim my Q. Yuumi's early game is very weak (naturally cuz enchanters of course), but with no proper coordination with the ADC, it's hell playing Yuumi cuz your heals and shields are very weak. Yuumi + an ADC is like Cho'gal from Heroes of the Storm. Only difference is you can pick Yuumi and make everybody Cho'Gal while HoTS, you're forced to have a premade to even be able to lock in that hero. You should see how that ogre even uses his mount...literally from parallel universe! hahaha Okay off topic, sorry...
Yuumi's source of her shielding is by damaging enemy champions, which basically means landing her Q (and ulti i think? Should also count), which can be too slow and difficult to provide. I think it might be a good idea if her shielding was on her W rather than passive, but that would make her without a passive (except that bonus AD and AP on ally, and mana restoration), so that idea is out of the question most likely.
As Yuumi scales and gets items, her healing is quite strong especially since it has 2 stacks. You can w and heal an ally and w another and heal and so on. She's a fun champion really, but most of that "fun" can be taken away if the ally is not playing properly especially with positioning, so it's a good idea to have a premade. Not much timing your abilities are needed except for maybe her R and W.
I think Yuumi is well designed for that matter, but does she need a buff or not? That's beyond me, and hell, it's too damn soon too. Only time will tell. Her mid game doesn't seem bad, especially her late, but her early? Yeah, difficult to say. Need to try and play her more and understand the champ better.