The majority of the bendbacks I've tied are two colors of bucktail, some flash, sometimes a topping of peacock herl & a body of crystal chenille or estaz, or sometimes a wrapped body of diamond braid or mylar tubing. I also add some grizzly hackle sides. Most have been in a size range from 2 to 3/0.
Occasionally I'll tie small ones as well, down to size 6, and with other types of hair such as arctic or red fox tail.
I've tied them in many color combinations but most of the time they're basic, such as chartreuse over white, or olive over white.
Although I don't think the fish care, I usually add eyes to them as well.
So far, I've not attempted them with synthetics.
I have a couple bendbacks that I purchased (got them cheap on Ebay with other flies in a lot) that were tied with a material similar to ultra hair, may even be ultra hair. I guess they'll catch some fish, but I haven't used them. My preference is for materials that have some movement to it, and some synthetics may be OK. I've used natural materials for so long on bendbacks, I'm reluctant to change.