I don't really have anything to add to the discussion but when has that ever been a deterrent to chiming in?
Several years ago I upgraded / upsized my shop compressor and put it into a doghouse outside the shop so new air lines were needed. At that time I researched what kind of piping to use for shop air and toyed with the idea of using PVC or CPVC water pipe for compressed air. The consensus was 'don't' because the stuff would explode and shatter if you bumped it under pressure. So at that time I went with copper and have been happy ever since.
Subsequently I saw the (what appears to be PEX) air line and fittings but was still leery about using it due to the warning about PVC and other plastic for air lines.
After that I re-plumbed some of the water lines in the house with PEX - and fell in love with the stuff. And boy is it tough! The only down side is that you shouldn't expose it to much sun- or UV light as that degrades the piping.
We're all pretty aware of the general quality level of HF goods.
So, long story short, if I had to do shop air again - or if I expand what's there - I'll do it with the air-rated plastic piping. Easy-peasy!
Stu