A plastic mainspring housing is a cost cutting measure. The fact some (as you have pointed out) owners have experienced failure in in this area is significant. I have heard of a few CZ polymer guide rods that show wear, but I have never actually heard of one wearing out, breaking, or being the source of a malfunction/failure. In reality, there is little stress or load on the polymer guide rod.
Everything in a CZ, to this day, is forged or cast. I don't see any real cost cutting in a CZ so far, unlike others.
