PoiDog, the ones that I saw, had all the change gears made of plastic, and a couple in the head, behind the cover. If a person could get them made of HS aluminum, brass, or bronze, then he would have something, and steel, even better. I don't think crashing one would hurt it, as keys should shear, if they are used with steel gears, or even a shear pin. The most that should happen is a stalled motor or a sheared key. The problem is, that plastic will not have the service life of something harder, so their teeth wear out much quicker. Keep them lubricated well, with a grease or oil made for plastics. It doesn't take much.
I have heard of the mini-mills heads going out due to the gearing, but I would say that they were taking heavy cuts with them. They have some larger bench mills out, now, that are made pretty good. I've been thinking about buying the combo horiz/vert bench mill they now have, for small parts.
I had one of those combo mill/drills at one time, and you could take a heavy cut with those. They were belt driven, but the spindle was an R8 taper. The spindle OD was about the same diameter as that of a smaller Bridgeport.