Whats the biggest problem with most fiberglass parts?

[BR3W CITY]

So I'm curious, whats the biggest problem people chase with fiberglass parts? Is it something on the user-end; hard to mount, tough to work with, changes in use/functionality? OR Is it things from the production end; poor quality bucks/molds leading to sloppy fit, extreme variation in panel gap, bad quality materials user or hugely varying quality etc?

I ask for a few reasons, among them being general curiosity and frustration. When I first did my body and then started to lighten the truck, I wanted a fiberglass hood and even moreso, I wanted a fiberglass tilt frontend. However it seems like almost no one makes high quality fiberglass for most trucks, let alone at a fair price.

For the 60-66's, US Body and whatever that other company was were the only options, and both only sold a few panels....and both were really freaking expensive...and both had terrible garbage quality.

I was shooting the sh!t with my old man in the shop the other day, and he rambled on about how he used to make fiberglass hoods and panels in shop class in the 70's (no doubt, he also walked uphill both ways in the snow with no shoes...) Anyway, he got on a boredom kick as many early retiree's do, and started poking back into the subject with a more fiscal interest.

So, back to the original question....what is the biggest problem with fiberglass race (and street I guess) parts for our trucks?