Hi there, I'm a D1 support player who mains enchanters such as Nami Sona Morg etc. My personal opinion after playing Yuumi is that she may suffer from balancing issues because she doesn't have very much skill expression. I think that her lane will struggle quite a bit at her current state but that her team fighting will be decent if your team isn't incredibly behind. This is fine really but I don't see anywhere in her kit that would differentiate between a 150 game Yuumi player and a 5 game Yuumi player since her abilities are pretty 1 dimensional.
I think Yuumi could be improved by gaining more incentives to play her unattached so that skill expression can be found in being properly unattached/attached at the right times.
1 example: Perhaps her Q could stun or microstun while unattached in the same line skillshot fashion. I think the goal overall should be to add windows where yuumi should choose to unattach opens her up to counterplay and allows balancing to be made around those kind of decisions. (a microstun or stun would also give yuumi some breathing room for lane and make her more viable in high elo). So your jungler is ganking, hop to your adc, speed them up, hop again to reposition to your jungler who is closer then hope out to land Q for example. This is much more high impact than whirling your q around to get a bit of dmg on them and a slow.
Her Q could also have splash DMG while attached which would also help strengthen Yuumi's lane the same way Karma can trade by splashing Q damage off of minions.
I'm sure there are more ways to achieve this but the goal is the same.
Unrelated but a balance idea: perhaps her ultimate damage could scale off of % healing increase OR mana regen that you get from ardent/mikaels so that it would be easier to balance around. Instead of all base stats/AP
Thanks for reading!