As for whether or not they are reliable, I would say (based on a lot of experience with small mechanisms of all sorts, and a fair amount of research into these FSU thingies) "Yes, they can be" with a bunch of caveats. Let's assume it was a good camera when it left the factory in Kiev in 1965. It is now almost 49 years old, you don't know how much - or how - it's been used, when or if it's ever seen servicing. I would expect to have to give the camera a complete overhaul, removing all old lubricants and replacing them with modern synthetic lubricants, not a job for the faint of heart - or the inexperienced. I've already done the same on a Contax II and it's involved to say the least. Expect to replace the shutter tapes as well. The lens may need internal cleaning, I recently bought a screwmount J-8 from 1975 and it had the stubbornest haze I've ever seen.
Any old camera you get will need servicing, it's standard maintenance for mechanical things. I usually buy things with this in mind, and I've yet to be dissappointed. The really really really clean Canon SIIb I bought needs a complete overhaul including new leather cover and new curtains (I knew this before I bought it), but I wouldn't have been able to afford it otherwise.