I've had the pleasure of being on the side of a Yuumi and on the receiving end and let me tell you. Yuumi IS FUCKING OPPRESSIVE. Laning is a chore because you can't stop her from landing the free poke on you. Her Q slow sets up practically everything for your ADC and auto wins any trade. Mid to Late Game her ultimate does so much that it practically wins any fight so long as her team focuses the rooted targets.
It's ridiculously overloaded and overpowered. Especially with ADC being a strong role at present. I'm not saying gut Yuumi but a few changes I would recommend are the following:
1. Increase the Q cooldown, Q mana cost, or both in equal proportions.
2. Remove the Adaptive Force Sharing on her W. I don't mind that it lets her latch onto a champion, shield them, heal them, whatever - we already have that with Taric's W and it's relatively healthy. But giving her AD free stats like that plus all the items she's building? It's like 2 v 1'ing a raid boss. It's not fair at any stage of the game.
3. Reduce the damage on her R. It has a root if it hits 3 times and the hitbox is huge. She can get the 3 hits. Reduce the damage per 2nd+ hit to 25%. She doesn't need the damage.
4. Make her Heal a one charge deal, but make it a little stronger to compensate.
This would balance Yuumi for both scenes.