Hey, I don't think just saying she's too strong in proplay is an accurate representation here.
For the upcoming nerfs it was discussed how Yuumi has an extremely high win rate growth on mains, with (if I remember correctly) 16% higher win rate after ~50 games.
This is giving these 'experienced' players a 60% win rate after the initial hotfix buffs which is a good enough reason for the current nerfs that were being looked at.
Please correct me if any of these figures are wrong, it's all from memory from what I have been reading about balancing patch discussions.