I say she's in a super good spot. I personally feel I can make her work in solo Q even. She might still be a little on the weak side, but I can usually make sure that 1-2 of her absolute hardest match ups are banned in champ select. You want to avoid enemy bot lanes with excessive amounts of front loaded base damage like Draven or Miss Fortune.
For enemy supports to avoid, Blitzkrank is the #1 worst match up to take Yuumi into if he's good. One pull and you waste everything on MAYBE keeping your adc alive. Thresh, Pyke, and Naut are equally as bad to fight, but I'm not seeing as much of them lately.
As Yuumi, if you can keep your adc alive, and not feed early, you WILL do well.
Edit: I run Guardian on her along with Revitalize and Bone plating so I can survive hopping off to recharge shield and often max E > W > R first and use Q purely for slow.
Encourage your adc to pick up Barrier while you run Heal, this creates the most benefit and all around sustain without the diminishing returns of multiple Heal summoners. Don't even bother with Flash, just run Exhaust or Ignite.