I have a Carbon 15 (pistol) with Poly upper and lower. Mine works fine and hasn't broken anything. Every now and then a Wolf steel case will stick a little in the chamber (I mostly shoot Wolf through it). I spray a little oil on the steel stuff. Brass works 100%.
A little history: Professional Ordnance came out with the Carbon 15 pistols to comply with the '94 AW ban. One of the more obscure provisions was that handguns couldn't weigh more than a specified amount. This was to discourage production of "Assault Pistols", like the Sites Spectre, AR pistols and other semi-auto version of sub-guns. So, Professional Ordnance figured out how to make an AR pistol light enough to comply with the ban.