I wrapped a set of headers back in the late 80's. Tehy did rust quite severly. They did reduce the underhood heat, a lot. In that car a 75 Bricklin, it was way worse than a jeep. Stick your head in an over for comparision. I then had the headers Jet Coated. It reduced the heat almost as much as the header wrap. They still look new. Although I haven't driven the vehicle much. Headers get hot, so I don't see why or how a bit more heat would crack them.
There are two types of Stainless, 300 series and 400 series. One if magnentic (the cheaper one), and the other non-magnetic. I think they use the magnetic one in headers, as it is easier to bend. And yes, I have seen SS rust. It was a revolver that was in a flooded basement.