Man, it feels like just yesterday everyone was saying Yuumi sucked...
Shes both hard to play individually and (especially) hard to play with as teammates, which makes for a double hitting learning curve. We've always known that played well she was strong, and after the hotfix buffs, we suspect that she was hilariously overpowered - but at the end of the day game balance must reflect actual players. We are okay with releasing champions a fair bit lower winrate than other champions currently are because players get better - but not 30% winrate.
And now that pro play is starting to kick in and people are starting to figure her out, we can start scaling back the hotfix buffs and refining her balance to be closer for the average player vs the coordinated pro team. Even then, it's entirely possible that some regions just have an incorrect read on her (ie: she doesn't seem to be nearly as high priority in China), so maybe shes not actually that bonkers. It'll certainly take time for this to fully settle, but we'll get there.
Fortunately, experience playing with Yuumi only increases with time, so in the future a new Yuumi player will both have seen other ppl playing her, and won't have to deal with allies who have no idea what she does.