I've broken a few of these also, still using the plastic ones though as they're so cheap.
I think I mentioned earlier in the thread that I had enlarged the drive pin on the center shaft as I'd stripped the slot on the spur twice.
You can see how much they hole has elongated on the steel drive shaft too, these shafts have an incredible amount of slop in them which is disappointing, I'm on my second steel shaft now!
The enlarged center drive pin still gave up eventually, so modified the gearbox to take this from a Tamiya. It's a 68t so needed a bit of space created for it but uses an aluminium hub. Running with an 18t steel pinion and the 23t mount meshes perfectly.
Shame I've melted the motor and esc now though, getting by on a little sensor less 13.5T and 45A esc for now, but it doesn't like 3S much until I can decide what to get.