Fiberglass is easier to crack as its less flexible, and as you know aftermarket replica parts aren't always 100% perfect fitment. With lips plastic lips you can bend and mold it with a heat gun to fit better whereas fiberglass you cant do that. Also, painted fiberglass parts tend to have a waviness within the paint. Some panel beaters actually go through the effort to get rid of this but most don't bother and just paint the part for you.