The early imported ones were pretty hit and miss but they got better. People figured out how to fix the "vodka specials" by driving off the gas block and adding or enlarging holes, as necessary. You can polish things up to improve reliability too. The goal is usually to get it to work reliably with cheap Wal Mart Federal bulk pack skeet rounds.
Mine was fine out of the box but I had to drill a couple extra gas holes when I SBS'd it to a 12" barrel. It doesn't run the cheapest stuff but it works 100% with low recoil 00 so thats good enough for me.
There is a S-12 forum that I learned a lot from but it's been pretty dead for a while because of the sanctions. The Vepr forum is more active now but I haven't been following it much since I finished that project.
Forums - forum.Saiga-12.com