I have only had two FSU cameras, a pair of feds which I had received in trade for my Canonet QL-17 GIII and a Seagull SA-85 TLR back in 2007.
The picture of the pair when I received them back in 2007.
2C Type 2A on left, and a 2A on the right. The left was working but was more or less as it was received by the owner. The one on the right was CLA'd by the previous owner. He made comments about how a lot of the FSU cameras seemed like they were serviced by tank mechanics wound up so tightly that it's like a gun going off when the shutter fires.
Said that he had properly tuned it, lubed it, adjusted it to be very smooth and even modified the shutter speed knob so that it could be operated regardless of whether or not the shutter was cocked. Meaning it wouldn't damage the mechanism if you turned it either way, you just simply wouldn't know what speed was selected until you cocked it.
The 2A that was serviced was the one I kept of the two along with that Industar-10 50/3.5 collapsible, and like he said it not only felt very solid but it seemed to operate very smoothly and accurately without feeling like a rat trap went off in your hand. Probably the best Leica styled rangefinder I have aside from the Canon P I owned and the Canon 7 I have now. My only gripe with it is that the viewfinder/RF spot isn't the greatest, but it's certainly usable and I have not needed to adjust it yet.
I never got around to using the Zorkis and similar, but I've had my fair share of playing with various Industar and Jupiter lens, the Jupiter-12 35/2.8 and Jupiter-11 135/4 being among my favorites (sadly I no longer have the J-12).
In a nutshell, with just my experience with the 2A that was professionally tuned, I feel like they can be tanks, but well-precisioned if work goes into it. But out in the wild (purchasing, not shooting), I'm reluctant to just pick one up because it seems like they can be hit-or-miss.