Boresnakes are great for light cleaning, like the cleaning after a small number of rounds. I don't think it would do great for a really dirty barrel. All cleaning methods have their pro's and con's, and this one is no different.
Bronze harder than steel? Son I don't know what gun you have, but if you find a gun barrel that is softer than the bronze brush used to clean it, you take that gun back, you bought the kiddie version. Ask them for the adult version. It'll have a steel barrel. The guns with the barrels softer than bronze tend to be bright colors and are typically sold with ammuntion in round sets of 8, and they go "bang" and sound like a 22. I recommend them for concealed carry, as there is no risk to life and limb when your gun shoots no projectiles! Hey, the BG doesn't know that!
Bronze harder than steel? I ain't buying it either. Actually, I would think nylon is pretty abrasive in its own right. You ever looked at a line guide on an old fishing reel? That "nylon" (monofilament) does a number on the metal after a while.