I wouldn't want mine out of aluminum.. it would get dinged by pebbles easily.
The problem is they used unprotected plastic for the part. ANY plastic will degrade over time when exposed to UV.
They should have primed and painted it with UV-resilient automotive paint and clear coat like the rest of the car. That's what I will do when I permanently paint my roof after I remove the wrap in couple more years. In the mean time, I have wrapped the part with Hexis cast vinyl in gloss black.
I wonder how much (if at all) several coats of synthetic paint sealant and carnuba might prevent the warping/cracking. Clearly the pieces are still going to expand and contract with temperature variance, but I have to think that a coating with good UV rejection will at a minimum prevent some degradation in the material.
I ordered some EXO from Gtechniq the other day because I've read really good things. By far the most expensive detailing product I've ever tried. However if it lives up to the hype (and perhaps prevents these cracking issues), it will be worth it.