I had a Zorki 4A that worked well once. It looked like it had never been used. Nearly every other FSU block camera I've owned (several) had some sort of problem or other. These can usually be cured by a good service of course. Only a few are really worth it however. One that comes to mind is the Pentacon Six (Praktisix) with its fine lenses.

I used to collect Praktina because some of the lenses made for it were often so amazing, but I could never rely on the bodies to work! Even after repeated servicing - drove me crazy.

Of course vintage Canon, Nikon, Leica etc. often needs work, but usually when fixed they stay fixed for a long time. More often than not however they still work well, even decades after manufacture.