Yuumi right now can just stick to anyone forever not taking any damage and heal away, that's great but where she is right now she can stick to a target and if that target gets caught out or otherwise assassinated she can usually circumvent it with her missing hp heal. Now, that's not the problem however i do feel like she needs to take a small percentage of damage dealt to the target she's attached to. This would encourage her to jump off her target more frequently to proc her passive and taking mitigated damage for being attached to the guy being bursted down.
Also, jesus boys her Q is maybe the worst most low impact ability in the game right now, no damage, low projectile speed lack luster slow and slow duration. Either give the spell the numbers to actually poke or use reliably as cc or change it entirely. As this ability stands right now it's totally unacceptable. And it's not like it's super easy to suspend the projectile for one second so that it gets the added effect either. I think you're definitely safe to reward a little more heavily for landing it.
TL:DR Make yuumi take a small amount of mitigated damage from her attachee
Buff Q substantially.