I have Super Rollexes in 6x7 and 6x9, a horseman 6x12, plus a graflex 6x9. These are all for my Technika V. I don't have any problem with film flatness from any of these backs, I use the 6x9 and 6x12 mostly, and sometimes with wide apertures.
The build quality certainly favours the Super Rollex, the other two seem a little flimsy in comparison, but in practise their is no problem at all. I have often looked at the Cambos, the idea of slotting them in like a double dark is very appealing, removing the GG to fit a roll back is a bit of a pain.