With Soraka, albeit annoying also, you can focus, you can dodge.
With Yummi, you can't.
She jumps on a champion, get's a mouse controlling 'skillshot' that is nearly undodgeable low range and heals like crazy without literary having any opening as a champion.
Sure it's incredibly dependent on the team. That's that champion's only weak point which can be mitigated with a duo.
What makes her even more broken is that if a juggernaut is slightly ahead and a tanky build, the game is 90% lost as he gets to be unkillable