I mean, in soloQ you can only count on yourself and your performance, so it is good to take Brand and Zyra, maybe Vel koz even, but not as good to take Yuumi. In duoQ however Yuumi might work pretty well. But you need an adc duo partner. Try to get diamond by playing Yuumi soloQ only. It should be impossible. But might be possible with Twitch jungle, even tho he is 45% winrate. It might be possible on support with Katarina support even. It is just about soloQ potential and Yuumi has a very bad soloQ potential. She is created for duoQ and maybe even for LCS.