[Perfectlap]

fiberglass is hit or miss.

Its only as good as the shop making it. I had a fiberglass aero kit on a previous kit made by Racing Beat who specializ in RX7 and Miatas and it was well made stuff. I spoke to someone who once worked there and they told me they threw out a great deal of product that didn't meet quality control standards. He said he bought allot of rejects for his own cars because it was a matter of minor blemishes imperfections that could have been fixed with a little time.
98% of Fiberglass makers will not be so attentive and will use as low grade materials as possible.
This means horrible fits where body panels don't line up properly and since most of these aftermarket parts are on sport suspensions they end up taking lots of punishment...paint starts to crack and looks like a wrinkled shirt in no time.

Now Composites like carbonfiber are another matters. It's a fairly low tech manufacturing procedure and risky for craftsmen to work with. Thus you don't see allot of genuinely well made carbon fiber. I was one of the first among my friends to buy an all carbon bicycle back in the very early 90's and it was actually made in Taiwan by Giant! I turned away from expensive aluminum because of its short shelf life. But as far automotive, it still seems to be standard for everything from ultra expensive racing helmets to mirrors.

Polyurethane, polyresins etc. used in factory aero parts from Porsche are well made, resilient and lazer cut so there's never an issue with fit. But its very expensive.