I was watching a video recently on YouTube showcasing special interactions between her ultimate and several champion skills.
Here's my position on the matter.
The idea that she becomes non targetable while casting her ultimate is very unorthodox, because as a movement displacement ability, it is natural that you can move her out of the animation via alternate displacement.
As for my opinion on some of these interactions.
Malphite and Vi both have "unstoppable Ultimates" meaning no amount of CC or terrain will stop it. Because essentially Camille's ultimate is a single player terrain, I feel it should pass through and cancel it via leaving the area. Because only a select few champions have such abilities it couldn't be deemed overpowered simply because only 2 champions can do it. As for champions like Rek'Sai and Jarvan IV where their ability interactions simply move their position, it would remain the same.
Another comparison would be to Azir's R - Emperor's Divide. It negates all movement through the selected area. This includes but is not limited to dashes, enemy displacement (Thresh, Blitzcrank, Janna), skill follow up (Lee Sin, Ivern). However it does not block or cancel Vi's R - Assault and Battery or Malphite's R - Unstoppable Force.
This is only an opinion. I'm no way shape or form do i insist it be done, but after observing several reactions between skills, my opinion has been set.
This is a discussion thread, so please feel free to add or comment on things I've said, but i urge you to not bring toxic behavior here.