Stainless has superior high temp corrosion resistance to mild steel - no question. As to why people have had trouble with it, all I can say is that it is rare to find that the material itself is at fault.....other factors such as poor design or poor welding are more likely causes. But, without an understanding of the cause, to blame the material is wrong.
MS is used because it is cheap, readily available and easy to work with. In this case, the cost diff between MS and SS is sufficiently small that I'm prepared to pay it for the extra benefit of having SS. Had the diff been greater the decision would not have been so simple.
I think I'll leave it there, I also don't want to get into a sf over this and people can make up there own mind.