Composite wood products have both advantages and disadvantages. On the advantages side, composites are light-weight, do not rot and composite "trim" products and many composite decking products can be cut, formed, and painted like wood. On the disadvantages side, however, is the matrix inside composite trims do not hold screws as tightly as wood does, so they're not always a good choice where a lot of flexing or stress occurs.
I used composite trim in one place on my trailer, for the frame I made for our roof-mounted
solar panels. Composites made sense in this application because the "wood" frame is constantly exposed to the elements, but to use them there I I used both screws and urethane glue to assemble the frame and mounted it to our rooftop with both silicone marine adhesive caulk designed to work on
fiberglass and six #10 2" coarse-thread stainless steel screws. It's been a solid and leak-free installation these last two years.
The inside of the trailer doesn't see the weather extremes that justify the use of composites, so I'd tend to assemble a well-fitting window frame from traditional wood products like poplar, hemlock, and other tight-grained but lightweight woods.