Just my anecdotes in case they are useful:
I tried 3 Pentacon Sixs. The first one worked perfectly (rare), the next two both had really big frame spacing issues and film counter issues, sometimes with an overlap that changed it to 6x3, more likely around 6x4.5. This persisted even after a CLA, the repairer told me film spacing can't be adjusted on them.
I tried a Kiev 60 which had mild frame spacing issues until it was serviced from Kiev USA. After that point it was flawless. That was 15 years ago, they may have retired.
I bought a Kiev 6C from somebody here on Photrio last year and it's been working well for 120 and 220 film. Before that I was using the Pentacon lenses mounted on a Mamiya 645. That worked fine, though auto-aperture is lost and it's a smaller format.