Yuumi apparently debuted on Summoner's Rift in a fairly underpowered state since her win rate was about 30 per cent recently. Riot Games were quick to react and implemented a hotfix that buffed several stats and all of her abilities.
Yuumi may be the first adorable cat in League of Legends but she also seemed to be the most underwhelming. According to LoLalytics, a website tracking champion stats, Yuumi kicked off with a miserable win rate that was the lowest ever since Syndra came out.
Granted, both champions introduced new mechanics that could have an adverse impact on the win rate until players learned them but Syndra never managed to become a powerhouse until a series of buffs were implemented. Riot seem to be content going down the same road with Yuumi, in hopes the tiny cat can become a better support champion.
All of Yuumi's hotfix buffs are listed on the 9.10 patch notes in the first section, dubbed Mid-Patch Updates. Buffs affected the base stats of all four of her abilities so Yuumi should heal and shield more as well as deal more damage and have a slightly longer slow. Some of the abilities also have better scaling now.
The same graphs from LoLalytics now suggest Yuumi's win rate is currently going up but it was still between 36 and 39 per cent at the time of writing. It is possible the champion is in a good spot after the buffs but player learning curve is keeping her win rate down.
Yuumi introduced a new mechanic that allows her to attach to a friendly champion and she becomes nearly invulnerable at that point which many players seem to do too much in bottom lane, leaving their AD carry as the sole focus of the opposing team.
It has been only two days since Yuumi was released so it may be too early to say anyone has mastered her, especially since the attach mechanic is more complicated than it initially seems to be.
Aphromoo found out the hard way that afk Yuumi is not a profitable playstyle. RIP assist.