Thanks for the reply! I do agree with a lot of your guys' ideas.
I think it's just a psychological sticking point, especially among those not in the top 5%+ of players, when it feels like you can't do anything to force a decision on her part.
It's an interesting thing how the skill of the players involved doesn't just change the power of the champ, but also fairly changes how the champ feels to play and play against. It doesn't seem to be that way with many champs to the same extent, but I feel a lot of champs that get complained about (warranted or not) tend to be champs that the majority of lesser skilled players feel bad playing because they feel that they don't have options.
That's particularly why I was just throwing out ideas for some hope of forcing interaction directly for players that are not constantly thinking about high-level champ mechanics like "weaving" Yuumi's passive for mana sustain and more thinking "Why can't I ever hit her! She's so unfun!". It wouldn't even have to be especially easy to do or consistent, as that would then make Yuumi feel powerless to stop enemies from knocking her off. Just something that is a "goal" an enemy could work towards to try to force that interaction in some way, simple and clear cut for both the Yuumi and the enemy to see as something to either go for or to avoid.
To tie in the incentives you mentioned so that some might enemies feel like they have some small amount of agency, just some ideas:
Maybe she gets some sort of bonus for the champ she's attached to the longer they are in combat, but 2/3 hard cc spells in quick succession knock her off.
Or perhaps she takes reduced AOE damage when attached, but if she takes a certain threshold of damage she is knocked off, making it riskier for her to sit on a tanky juggernaut but still able to pull it off if she times it properly. Enemy champs would have to choose between focusing down the big bad motherfudger before she jumps on, or waiting to bait her in and knock her off in the middle of their team.
Or perhaps she loses healing potency on the same target multiple times in a row but gets a bonus when she jumps to a new target, making the "I sit AFK on a melee carry and press e" playstyle.
Or maybe she has a slight delay when jumping from or jumping to an ally champ where she is still targetable, allowing quick reflexes or predictions from the enemy team to pull her off or at least put a bit of poke on, forcing her to either stay on a particular champ or jump off to use her passive and be able to heal herself back up. She wouldn't feel any different to play with mechanical timings, but jumping onto a champ in the middle of the enemy team would be much riskier, but still possible if timed well. Again, the enemy team would have pretty clear decisions to make around this small change.
Anyways, ignore my washed-up former plat redditer ramblings as they probably aren't worth much. But thanks for taking the time to read and reply, and I hope you guys can find a way to reach your goals with her :)
P.S. If you use any of my ideas I reserve the right to feel privately smug and add it to my fake internet point bank vault. And if you don't well... that's probably a better choice for you haha.