The gear is plastic. I suspect the problems of the original spur were due to the layshaft/pin system and the moulding on the spur, causing a weak point.
I sadly have not had opportunity to test the slipper. Bad weather has curtailed all that
The metal spur gear should be a direct fit into the slipper though looking at it, so even if the plastic spur splits, all should be good. Of course you could fit a nylon spur and things should work ok.
The best news I found was it was an easy and direct fit. Now you have the slipper to correct overpower from the motor, then combined with a decent wheelie bar should sort out any lift up wheelie from the bumps. The silvercan I still use wont lift the front end on it's own, but 5000kv+ will, so It's an easy and cheap upgrade to fine tune that last nth degree.