More reliable? Over the lifetime, probably decent but no, not as much - only because there are more moving parts that are intricately involved with one another - especially a gas gun. That said, will a GOOD gas gun go hundreds of thousands? It should, and easily
Is it a fair statement to say that a better quality $600 - $800 semi-auto shotgun can be bought more so than an O/U in the same price range?
Without question. An 800 semi, will most likely be better than a 1200 O/U.
Pumps and semis barrels are the main cost difference between guns with two barrels