It all depends upon application. Alum.frames seems to work just fine in .22 LR guns, many centerfire pistols and even, some revolvers, if I recall it correctly. Polymer frames are used in centerfire pistols, but never saw anybody trying to use polymer for full powered rifles or revlovers. Steel can handle anything, from .22 LR to 18" battleship main guns. If one looks at mechanical properties ultimate/yield/fatique strength of typical polymer used for frames, these numbers are below similar properties for aluminum and on average, 7 times below numbers for even the low-grade steel...However, alum frames can be easily damaged if droped, and alum. alloys are not know to provide good wear resistance where pure friction wear is present. On the other hand, there are some alum. alloys with
silicone, which are super tough, but it's a hell to machine those...
What I'm saying is go with all steel guns and don't worry about nothing.