Real Dremel nylon brushes are of good quality. The strands are well mounted and don't readily pull/fall out but will eventually break off with age or hard bending use. Think of these brushes like miniature tooth brushes.
The brushes themselves are minimally abrasive and provide primarily physical force for fine cleaning using water-based soaps and cleaners. Nylon appears to stand up to mineral spirits and some gun cleaning solvents well but is degraded by acetone and lacquer thinner as well as hydrogen peroxide and many acids.