I think Yuumi needs a buff on his basic attack/attack speed
Litereally a minion can hurt more than him, as an champion that have a passive working around him using basic attacks on enemy champions, I can say the shield is not worth how much hp he will lose doing it. Yuumi is probably have the lowest stats compared to anything, and I understand why, being able to be in a zhonya all game while also being useful is cancerous as heck, but when he uses his basics he is as vulnerable as anyone, maybe even more since he can die instantly without someone to attach to.
So that is just my opinion and low arggument, but his raw stats are extremly meh, and supports usually depend on their base stats and flat ones.