There are different qualities of rayon thread, just as with any other thread. Both raw materials and manufacturing methods affect the characteristics of thread.
One thing to be aware of is that rayon thread will often lose its color if exposed to bleach.
Some embroiderers prefer rayon because of the end product, some prefer polyester, some prefer one over the other depending on application.
I found a few websites with opinions and information, if anyone is interested:
http://www.connectingthreads.com/tut...read__D89.html
https://www.superiorthreads.com/educ...l-about-rayon/