Cyclops nozzles are currently only available in Brass.
General purpose, low cost, great for printing materials that don't have anything abrasive in them. Brass has a great balance of properties; thermally conductive, machines precisely and easily even with very small nozzles, doesn't corrode or pit so retains a smooth surface finish for cleanly laying down extruded filament.
Vulnerable to abrasive erosion by more exotic filaments like carbon-fibre filled materials, metal powder filled materials and glow in the dark. Can be damaged by nasty head crashes with things like bulldog clips and glass. Using a wire brush on a brass nozzle will cause wear over time.