Stainless steel is not exactly superior, it has its good aspects and its bad. Stainless steel, being much harder, tears up fret files, and luthiers tend to charge more as a result because they have to replace their files more quickly, and it takes more work when working on the frets.
Yes. The traditional nickel silver, being a bit softer, has a bit darker sound in comparison. For instance, a maple fingerboard Strat with nickel silver frets sounds like we'd expect, a little bright, but switch to stainless steel, and it'll be even brighter. Is that good? Probably not if you're looking for a guitar that has a traditional sound, like an artist or custom shop guitar.
PS - I have stainless steel frets on one of my Strats, which also has a roasted maple neck and fingerboard. That guitar is much brighter than my other Strat which has a regular maple neck and rosewood fingerboard and nickel silver frets.