Here's how I see it.......... taps and dies are steel, most of the material we use (ebonite, acrylic, alumilite, true stone, lucite, m3, etc) are basically soft material and ALL can be threaded, given the proper technique, tapping solutions and acquired skill.
I wouldn't say polyester resin can't be threaded or is hard to thread because all polyester isn't the same, the same is true with some of the other materials mentioned. Some polyester blanks are brittle and poor candidates for threading because the people making them don't know what they are doing, others do take the time to learn proper mixing ratios and their polyester blanks can be threaded easily!
Because of personal experiences and customer feedback, about the only materials I won't thread are Inlace Acrylester, m3 and true stone because it's been my experience that while they will take threads, the threads crack easily or wear down over a short time period of daily use. My hands down favorites are alumilite then ebonite.