I bought and returned 6 M6 bodies to get the two that I have that actually work correctly. All the others had serious issues ranging from inaccurate focus, oil on the shutter curtains, misaligned rangefinders, and one had haze in the small RF window!
When I bought the M4 bodies that I had, I originally wanted an M2. After going through 6 of them, all returned, I gave up on finding one with a rangefinder that wasn't dim and impossible to focus. I had to return 2 of the M4 bodies I bought before finally getting the 2 good ones I had.
I've looked at a few in camera stores too where I could touch and look. I bought one of my M6 bodies at a store here in Indiana that had the M6 I bought plus an M4, an M5 and an M2. The M6 was the only one not obviously broken! The M2 rangefinder was extremely dim. The M4 rangefinder was way off, I focused on something about 4 feet away and the lens indicated 25 feet! The M5's framelines were screwed up and didn't change properly when different lenses were mounted.
Why so many bad ones? First of all, these were all, except the M6's, very old cameras. Antiques really. They probably hadn't been used in a long time and were gummed up. The M2 finders are probably all just shot from aging of the glues used in the prisms in them. Old cameras need regular use and regular maintenence. I am always amazed at the "I've had my camera for 50 yrs and by God I never had to CLA it like you fools!" posts. Do you never change the oil in your car?