There have been many champions that I hate (and so do you) like Yasuo, Zoe, Riven, but we manage to live with them and accept them into the game. Yuumi however, is beyond broken in her current state. She is a Janna and a Sona who provides **insane** healing, AoE root and extremely long range slow. And the best part? You can't even kill her. She will just latch onto the carry/the champion with the most kills and you're done, there is nothing you can do. With a Soraka, the process is simple, you focus her at all cost and get the rest. Yuumi, haha, good luck.
The past couple of games I have played, the team where Yuumi is picked, is the one that wins, period.
Something needs to happen, where maybe if she heals the same target twice in one sitting, the heal amount is lowered, kinda like how PTA works, but it heals, not damages less every time.
Edit: If you're going to downvote the post, at least post some constructive criticism in the replies below, and spare yourself likelihood of me interpreting the downvotes as that of anguished Yuumi mains.