Hi,
If people fussed over them and spent as much time/money on them as they do on other maker's cameras, all of them would be great. Luckily they are nowhere near as expensive to repair and get into fine working order as other cameras. That's based on my experience as the owner and user of several Leicas and a Contax of roughly the same age as the ex-USSR ones.
The trouble is a lot of people judge them after buying a secondhand one and expecting it to work perfectly. Old cameras are not like that and often need a little TLC before giving their best again. Once put back into full working order there's little to distinguish my Leica II from my FED 1. And I could tell some horror stories about Leicas bought for serious money from serious dealers.
This is true up to a point.
Some FSU cameras, particularly Leica clones manufactured in the 1950's can be reliable, if somewhat crude, shooters, even as received. Others, manufactured in the 1970's and later, were badly made and hopeless junk; servicing these is a hopeless waste of time and money.
My own experience has been favorable with Zorki 1's and the Zorki 3. The Zorki 3 especially was a nice useable camera. These are the only two FSU cameras I would consider again. Later model Feds and Zorkis were troublesome with issues such as light leaks.
I had nothing but unhappy experience with Kiev 4's where the complicated shutter and poor manufacturing prevented these from ever being useful. This after two trips to Oleg for repair. I like the feel of a Kiev/Contax but never again. The fit and finish of these is a good indication of what was inside.
Regarding lenses, I've found the Jupiter 8 and Jupiter 12's to be entirely useable. A potential issue with the silver J-8's is that the helicoid lubricant often solidifies and the lens need be dismantled to relube.
My J-3's were fine after I had them shimmed, about 50% of them needed reshimming.
The Industar 50mm f3.5 has the finest build quality of any Russian lens that I've used, really well-made things. I've tested a number of these photographing newspaper targets on a wall and found that these were all OK lenses but none of them (12 or so) were quite as sharp as any Elmar 50mm f3.5 that I tested.