Yeah it's okay, just be careful especially if you are using a rotary or even a DA because the plastic will heat up a lot faster than the metal body panels. Other than that it's pretty much the same as polishing normal paint.
Pro Tip: Stop every once in a while and put your hand on the section you are working on. If it's super hot consider trying a lower speed setting/less pressure/faster arm speed, etc. Also definitely work in the shade.
Or, if you are really worried about it you could always just do the bumpers by hand...