Only Piko and Gützold (still) use plastic for outside motion. That's been a thing ever since the 1970s when the East Germans had to make the best of whatever resources were available. I'm not sure what grade of plastic they use, but they have perfected the method over the years and, in the rare event of anything breaking, spares are cheaper to buy. I've only ever had to replace one coupling rod on a Gützold DR Br65.10 2-8-4T, costing no more than a fiver.
The "clip-fit" technique makes the motion parts easy to fit, with no riveting required.
The upside of all that was that the shop prices were about a third to a half of their Western rivals, so offered UK modellers (traditionally notorious for short arms and deep pockets) a cheap way into Continental HO. I remember testing a Piko Br.03 Reko which was priced here in 1985 for thirty quid brand-new. The nearest Western equivalent, a Fleischmann Br.01 (they didn't start making 03s until later) was already above £70.