the lower plastics are the SAME plastic that the bumpers are made out of, the lower plastics are actually even PART OF the bumper covers, so, it can definatley be painted to be just as durable as the bumpers.
they do take a bit of prep work to really get them right though. first thing is, you can never do a good job with any of these parts on the car... impossible. second thing is they do have a texture which has to be smoothed for it to look right, means a bit of labor to really smooth all of this area. then, they need to be treated just like a raw bumper, meaning adhiesion promoter, good flexable primer, flex added paint & clear etc. idealy the whole bumper should be sprayed to avoid the seem being a potential problem area but it can be done as just the lower bumpers too without too much comprimise
it is not a cheap procedure... most times guys don't want to spend what it would really take to do it right... many of the cars you see with lower trim sprayed like that the guys want it done for just a few hundred, so they just tape it up scuff it & spray it... crap. peals & looks bad, that's why some shops will tell you it won't last cause they aren't going to do it right anyway...