Rayon thread might fade in time and as someone else mentioned not as strong. I also embroider but use polyester threads. Have used these threads on my Innova quilt machine with great results. Also my metallic threads are also embroidery thread purchased from BFCreation as my brand of embroidery thread has been sold out to Floriani and their prices have doubled so going to switch to BFCreation's brand when I start to run out. I purchased most of my dealer's stock of the colors I used the most so I'm good for quite some time.
Serger threads are about the only ones I know never to use as they are weaker than most threads. Maybe that's why they use 3-8 threads at a time.