Yeah I've read that about the film-flange distance as well. But its just internet hogwash. A difference of 0.20mm in the film-flange is actually huge, and using Leica spec lenses on an M-mount camera with a film-flange distance of 28mm would result in such severe front focusing that even the most laymen of laymen would notice something was wrong. Here's why.
On a Nikon rangefinder, the correct film-flange is 34.50mm and there are shims in between the focus helicoid and the camera body to ensure the right spacing. I once experimented and removed the shims and took some photos. Severe back focusing. I then installed 0.10mm shims. Focus was good. I then swapped the shims for 0.20mm shims. I got bad front focusing. I then added the 0.10mm shims for a total of 0.30mm of shims (0.20mm more than needed). The front focusing was so bad the camera was unusable. That was on a camera with a register (film-flange distance) of 34.50mm. On a camera that is supposed to have a register of 27.80mm but is actually 28mm, the focus error would be even worse.
I suspect the Hexar RF got a bad rap initially because of a few slightly misaligned RFs, and because well, its not a Leica
Edited to add: you can actually prove this for yourself. Add 0.20mm of metal tape to the mount on your M-mount camera. Attach a lens, and then adjust the RF so that infinity lines up. Go take some photos and look at the results. I guarantee you that nothing will be in focus.
I alos just remembered that Amedeo, the guy that makes the Nikon to Leica and Contax to Leica adapters, once told me that he has to get the thickness of his adapters to within 0.05mm of spec, otherwise they just won't work.
