Polyester thread is commonly used for quilting. Threads have come a long way over the years. Back in the 70’s invisible thread was stiff, poky brittle, fishing line- Horrible to work with & yellowed & broke over time. Now invisible threads are wonderful, soft, pliable, do not yellow are not poky are nice to work with. People 25-30 years ago may have had bad experiences with polyester thread but the threads of today are just so much better.