[ElevenLathe]

That's the idea (I assume, not the same guy): you're right that is mostly the form of the part that makes it easy/hard to work, not the material, but nobody designs machinable plastic parts. they design injection-moldable ones. So in practice "plastic part" is synonymous with "impossible to recreate in small quantities". Yes, in theory plastic parts can be of the same shape (and therefore ease of boutique manufacturing) as metal ones, but they aren't, not in the real world.