Hey folks, film flatness is a Rollei is nonexistent IF you don't leave film in the camera for weeks or months. It's that simple.
With only 12 exposures to play with on a roll of 120 film, just plan to shoot the dozen, and remove the film.
I've had Rolleis since 1966. For what it's worth, my 3.5 E2 (bought 48 years ago this month, and till in use) has had to be serviced 2-3 times for film feeder issues, but it's an easy fix. My two Rolleiflex Ts (bought in 2006 and 2008, they were such good deals I couldn't pass them up, and am happy I didn't) just feed film directly ie no feeders (rollers), and I've never had a problem with them. A recently acquired 2.8 E was in storage for 35 years, oddly the shutter speeds work perfectly but everything else is as stiff as I am at almost 70! It's set to go for a full CLA next month which will include a check (and probably adjust) of the feeders.
In my almost 50 years of shooting with Rolleis, I've owned a fair few but these are my keepers. I've found that nothing much can go wrong with them if you don't drop them, if you do they magically rotate in the air and always always always land on their viewfinders, with horrible results.
Whichever ROllei you buy, budget for a CLA and then use it. You will love it.