There are a lot of different "stainless" steels, too. Technically, they're referred to as CRES, corrosion resistant steel, and underground is a very harsh environment for any ferrous alloy.
One thing they have in common, though, is that they're all going to corrode slower than ordinary carbon steel. I would not be surprised to see some grades of CRES rust out before a copper clad rod, but I'd expect every single one to outlast a piece of rebar.
Can you post some more information about the studies you're referring to?
