I'm still using steel .45 ACP cases from WW2. I reload 'em 'till they crack, and then I throw 'em away.
(You really need a carbide sizing die to reload steel cases, though.)
I think that the problem is not the steel cases, but rather that some bullets are jacketed in "mild" steel.
Steel-jacketed bullets may be bad for your gun's barrel, even though they are plated with a softer metal surface.