Brass is metal, I'm assuming you're comparing brass and steel. Steel cases form a worse seal in the chamber of most firearms compared to brass and therefore allow more gas and fouling to enter the chamber. Because of this it tends get guns dirty more quickly potentially decreasing reliable feeding/extraction, but generally no real damage.

Hi Donald and welcome. Not much more. Steel does not expand as much as brass which can make some firearms not cycle it as reliably. Most modern firearms will have no issues. Cheers and stay safe!