Stainless steels, specifically austentitic stainless steels, have a much greater thermal expansion rate than carbon steels. Secondly, nearly all domestic water supplies contain chloride ions.Thirdly stainless steels have a much lower thermal conductivity than carbon steels, which aggravates the problem of thermal stresses due to expansion. Fourthly Austenitic SS's have a lower yield strength than carbon steels, so tubes must have thicker walls.............which needless to say results in greater thermal stresses.
The combination of elevated temperatures, high thermal gradients in the tube walls resulting in extreme thermal stresses, and chloride ions, almost invariably leads to stress-corrosion cracking (SCC) in SS. This leads to poor, unpredictable service life compared to less expensive carbon steels.
As I'm aware, certain duplex stainless steels may be used for this purpose, since duplex SS has a lower expansion rate, better strength at elevated temps. better heat conductivity, and lower susceptibility to SCC. For example SAE type 2507. I believe in some instances nickel super-alloys have been used for this purpose, though I'm not aware if they are cost effective in terms of service life versus their extremely high cost of materials and fabrication. I'm not an expert in this regard. I may be confusing materials for boiler tubes compared to materials appropriate for dry steam superheater tubes. Nickel superalloys would probably be a good choice for superheaters.