Just being stainless steel is no guarantee it won't rust.
Wikipedia it to see what it's made of and the different grades of stainless out there. To be frank, only crap stainless rusts, regardless of the sticker price of the item.
Milltek exhausts used to rust when I used them years ago, and the tube wall thickness was so thin, they'd crack as well. So since then I've stuck to only OEM exhausts or brands I trust like SuperSprint, Borla and Magnaflow.