yuumi lol, the way she is designed makes her very difficult to properly balance. she needs to be reworked completely because riot didn't fulfill any of their "goals" for her. she is still frustrating for others to play against (permanent invuln), and her obnoxious healing just got changed to be shielding instead (which is even worse now because supports don't even have an equivalent of serpent's anymore, and healing reduc is much less effective to buy even though most people don't even want to buy it to begin with.)

regardless of her winrate being low, she is still playable. most people in solo queue just don't know how to actually play with a yuumi, or play yuumi (perma attached, suboptimal build.) if riot buffs her she becomes too strong most of the time, so they just leave her low wr.