Metal spur gears are common, you will find the clutch bell housing is usually hardened, on such machines, also the spur gear is usually narrow, saving weight, there is nothing wrong with metal spur gears, some dont even have slipper clutches, in those cases, usually the narrow toothed spur gear will still chew out first, as it IS designed to do, this still requires more torque to chew out than a plastic nylon type, so the metal ones are stronger, by a long way, but, when they do finally go they, like someone said, make a jolly right old mess of all surrounding cogs !
My zircon l ran metal spur gear for 2 years never had one issue with it, got alot of wear on it now and l wouldnt use it anymore, the zircon is a 1/8th and about as big as a LSD2 losi , only heavier !