Actually I have done research and can back up my statements... some of the materials I have worked with needed to survive hot gas flow in a jet turbine in environments where sea spray was present. I also lived in upstate NY (Albany area) and the stuff used on the roads is the same or worse than the stuff you get in the midwest. The salts usually contain a mixture of chloride and sulfates plus some organic counterions...
The problem with most stainless steel exhausts is that they are not made from stainless steel (chrome based 3xx series) but are corrosion resistant steels. The oxide layer has greater solubility than chrome oxide, especialy when chloride or sulfate are present, and dissolves at a slow but finite rate.
The second problem (which I noted) is that the welds frequently have less corrosion resistance than the steel tubing. Once you crack a joint or two the exhaust either needs an expensive repair or replacing.
Most of the problems with 'stainless steel' exhausts failing have to do with the manufacturer using a corrosion resistant alloy or aluminized steel. The terms stainless and corrosion resistant steels are frequently confused by manufacturers. The customer buys the 'stainless' exhaust figuring it will last the life of the car but it fails in 3-6 years in a harsh environment.
A true 304 or 316 stainless exhaust system with stainless welds will survive 10 or more years in the harsh environment you describe. It is unlikely the system you described was manufacturered using a chrome based stainless. It may have been manufactured from a nickel based (4xx series) low chrome alloy described in a previous post, and the results were predictable.
