I will say this, I have seen multiple Trex decks with mildew/mold problems.
About 4 weeks ago, we ripped down a deck with moldy Trex, that was built by a home owner. (The deck was not built to code, and the decking was unsalvagable due to the mold).
Neither I, nor any fellow GC's that I know and deal with, will recommend Trex.
I especially, would not recommend it in any areas that are prone to heavier amounts of dampness and moisture.
Here's a close-up shot of the Trex boards on the deck we demo'd. The black spots (speckled all over) are the mold. There was more on the main area of the "deck":