devtrackers.gg
I hope you see this comment: have you folks looked at a very similar character from dawngate called Mina(i think)? She also became untargetable by attaching to her allies, but she took a percent of damage they took, and was encouraged to detach with cc on detatching. Obviously yuumi can't be the exact same as she currently exists, but mina never felt as bad to play against because you knew you were punishing her when you hit her host.
Yes, of course we considered and even tested a number of mechanics like this, both in original develompent and when I was making the large set of balance changes to Yuumi. Mina came up in conversation more than once, as well. They generally felt absolutely terrible to play, and generally betrayed the core conceit of the champion. They also often led to a lot of resentment for the person you were on, because they were actively killing you with everything they failed to dodge, and that was a pretty huge miss on the feel we were looking for. Ultimately, my takeaway was: was a fine path to look at, glad we tried it, it really was not even close to working in any incarnation we tested.
I think a totally different champion could probably exist with a mechanic like this, but Yuumi is quite far off from being that champ and (more speculatively) that champ also likely doesn't hit the same audiences that we aimed for with Yuumi. That's a good question though, thank you.