Here is my two bob’s worth on this subject. First let me declare my position so that I may be accused of bias, which is only partially true. I supply decking materials from my own factory in China, but I have had quite a bit to do with composite products here because I have clients who have asked me to check them out. There are many manufacturers here.
The difficulties with composite decking material come down to two issues; ingress of moisture and UV degradation. The UV issue is the easiest to address – in theory. You simply add more UV blocker to the formula. However, the difficulty is knowing whether or not your supplier is doing this. The blocker is expensive and so there is a temptation in a highly competitive market to cheat on this.
On the moisture ingress problem, it comes down to both the formula of the resin and the kind and volume of wood powder used as filler. This is not an area where I can comment in detail, but different fillers have different propensities to absorb moisture into the composite and also different properties of expansion and contraction. What I can say is that it is very hard to know whether the factory is doing the right thing or not.
To balance things up, regarding wood that it is not all good news. Whatever the species no sapwood is durable, for example, and no wood is without its particular characteristics. However, the advantage with wood is that the “formula” is knowable i.e. you can research a species and have a fair idea about its properties and so you can reduce risk to known parameters. You don’t need to particularly know and trust the supplier.