I don't know whether this is a real problem or not, or whether there is something wrong with my gear, but here's an experience:
When my first Bessa L died, after just a few rolls of film, I thought of replacing it with an FSU body. So I bought a FED 3. I quickly gave up, since the lens wouldn't thread into the body all the way; it seems the Russians and Cosina have different ideas what angle the lens should tighten at. The "top" of the lens was canted about 15 degrees when threaded tight.
Also, my FED has proved to have so many problems (pinhole in shutter curtain, other light leaks, scratches film) that it became a paperweight. I ended up buying another Bessa L, and it has worked flawlessly for a couple of years now.