We're talking about Yuumi because of these changes. The problem is that Yuumi can play really safe and scale for free with very little lane interaction for her opponents, when she's a late game enchanter, so she's impossible to balance
If she had a weakness or there was a way to play around her, she would receive compensation buffs to be able to lane and be a reasonable champion when not attached
Yuumi should attach to buff her ADC and clutch heal them, help them kite/chase. If she fails at her job and a melee champion gets to auto her ADC, there should be some cost to that. If her ADC can't dodge skillshots with the movement speed buff, I also think that should come at a cost
Now there are some things like Katarina can just shunpo auto, Zed can WW auto, fiddle/warwick can flash fear... but if Yuumi is also an actual champion while not attached it won't be such a bad thing. She won't be attached, but she'll be able to function and do stuff, then attach to someone else or her ADC if they live
I think it's not necessary to detach her like this if she's attached to a melee champion, who will naturally go into melee range of other melee champs and try to tank some damage