1) Rotom-C 2x resists it, not 4x.And Rotom-C could set up on those bulky waters, since it 4x resists it?
Just a thought.
2) Rotom-C's only set-up move is Charge Beam, which isn't that great anyway.
3) They wouldn't stay in as it sets up, for fear of Leaf Storm.
4) Rotom-C has a deathly fear of Toxic, so it'll have to be careful switching in.
6) Every Pokemon gets beaten by something. For example, Rotom-W beats Excadrill. Doesn't mean that Excadrill isn't a fantastic Pokemon.
7) They aren't even they only ones beaten. Rotom-C beats almost all Water-types in the tier.