Verified answer

No

Honestly if the fiberglass is done half decent it is a very resiliant material and will hold up really well if treated when it was formed. Just make sure the thickness of the fiberglass is enough to hold the speaker weight if you intend to attach the speaker to the fiberglass itself. Other cracking would really only happen if there was a pretty severe amount of pressure put onto it. Corvette bodies are fiberglass for example, and they hold up quite well to the elements so long as the paint stays on :)

That being said many of the top car audio system builds use a lot of fiberglass in their setups, including fiberglass sub enclosures (a good measure of durability) but keep in mind that the fiberglass work is a pain...if doing yourself and takes a good deal of time to make sure everything fits like it should and doesnt look sloppy. Just be prepared haha, good times.