Brass wool is often just copper plated steel, and will damage the plating. Thus, pin holes in the tips iron, nickel, or chrome plating begin to pit the copper core.

https://www.eevblog.com/forum/beginners/brass-sponge-has-rui...

Anecdotally, all wool seems to accumulate abrasive crud, various amalgamated solder waste, melted plastics, and trimmed bits of steel etc.

Best of luck =3