By the way, go by the local playground and see what kind of shape the plastic equipment is in.
Automobile dash boards are designed to resist sunlight, but I've seen far too many to count that had to be replaced after a few years in the Florida sun.
I've also seen many plastic parts and such on boats that would fall apart in your hands after a few years.
Broken up plastic lawn chairs and play ground equipment are a common sight at the dump.
But the point that few handguns are exposed to sunlight for hours on end is valid.
The black pigment normaly used by Glock is UV resistent, I have several RV bins of a similar plastic that have withstood near ten years of exposure to the sun while other bins a few feet away have become so brittle they shattered when I went to move them.
They sand colored frames are supposed to be okay now, but they haven't been in widespread use long enough to say for sure if they'd hold up to tropical sun.
Another potential problem of the glock is how they come apart when a casehead ruptures. A P-35 or 1911 with a casehead blow out will usually require a new magazine and new pair of grips, while a Glock frame/grip will shatter like a jelly jar in your hand.