You can love or hate her, but Yuumi is often picked in soloQ games and competitive matches of League of Legends. Whereas she had a 30% pick rate at her release, the changes implemented in the last patch and the hotfix some days after her release turned her into a more popular champion. She currently has a 46% win rate (which is still quite weak).
Even with this 46% win rate, Riot Games have decided to nerf the champion with the next patch (9.12).
We can say that playing against Yuumi is frustrating. The champion is safe and also has great mobility. Her Q is hard to dodge when she's in good hands and you can't touch her as long as she has an ally alive (who she will heal as much as possible). Pro players understood quickly those advantages and pick her to slow the opponents down and chase them more easily.
Base HP upgraded from 432.36 to 480
Mana regeneration reduced from 40-160 to 30-160
Opponents now grab one assist on Yuumi if they attack the ally to which she's linked
You can now be interrupted during your dash by any CC
Bonus changed from 5/9/13/17/21% to 5/7/9/11/13 + 4/7/10/13/16%
Those changes are currently on the PBE and should come with the 9.12 cycle.
Written by Charlotte "Cthulhu" Rognant. Translated from French by Millenium.us.org.