I've made several relatively small projects out of that plastic decking material. It is not inexpensive yet less so than teak, of course. You don't paint
it because it weathers into a greyish finish that looks much like weathered teak without the disadvantage of weathered teak. Weathered teak is not as strong as finished teak.
The decking material is easy to work yet should be used in compressive loading applications, not tension loading. The material will not hold fine threads well, only coarse realtively deep ones like the #12 and larger self-tapping screws.
If you actually make a cap rail out of the stuff you will need to through-bolt the material using washers under the heads of the fasteners. You can still counter bore holes and use plug
cutters to make plugs out of the stuff to fill the counterbores.
There is one DANGER! This material is VERY flammable. Light a small piece of it and you will see what I mean.
It is easily damaged if you drop something on it yet easily repaired as well.