Yuumi feels like the only true support
Every other "support" basically does their own thing and you have to react to capitalize on it. With Yuumi supporting you, you are the one essentially in charge of every thing, she just augments how you do it.
That being said, talking about Yuumi, I don't think her untargetability is an issue. The problem is that too many unskilled players are not used to the concept of games having different elements, they would prefer to play "their own game" rather than the game that they are actually in.

can because of his mobility
can because they have good roams
are all capable of being a dynamic support
Just because Yuumi can just decide to sit attached to someone so she herself doesn't have to position properly doesn't mean shes better than anything at all