This suggests that fibreglass is susceptible to abrasion by sand. Possibly not the case with polyethylene?
Every material is subject to abrasion by sand, some materials are just more susceptible than others. Fiberglass is much harder than polyethylene and therefore much less susceptible to abrasion. Try cutting fiberglass with a knife, you just cannot do it but plastic is very easy to cut.
I bought a used paddle kayak a couple of years ago. It has a couple of shallow cuts on the bottom, near the bow, where it was probably beached on a sharp oyster shell, or something similar. That would never happen with fiberglass.
I have decided that for my AI I am going to get a "dragging tarp" that I can strap under one end of the yak. I will use that to drag it over the sand or grass when I am launching it. But I will be extra careful never to drag it over a beach of shells or rocks. It could still get cut.