The plastic is fragile, but not brittle. Miniatures of no type are made to withstand any kind of impact. Metal ones are worse, actually. Plastic miniatures only break from an impact, metal ones can distort. You can't glue back on a smashed face. These would be the same as if you purchased a new Reaper miniature, and would likely require cleaning and preparation to be paint-ready. The rule with miniatures is that you store them only with miniatures of the same material (if you care). When friction enters play, the softest material will always yield first, so storing these with metal miniatures is a bad idea.
...normally produce just prototypes...
Exactly correct. They produce a master in printed material, and then have it squash-molded in rubber and a master is made out of brass or bronze. The master is replicated enough times to create a production mold. I have barely any idea why they use brass instead of white metal for plastic masters--they just do.
Goblin-King wrote:People paying $25 for a custom mini
Wait 2 years
People paying $5 for custom mini

You don't play much D&D, do you? I have customized hundreds of miniatures for PC's. There's just no waiting.
You're right, though, in two years' time the cost will be down (maybe not $5 down). Bones is the only way Reaper can compete with this, actually. Sculptors that I know are moving into digital left and right.