I am not sure what you mean by "Marine Polymer wood" but in a general sense, most plastics wood substitutes tend to be weaker and more flexible than teak. I have replaced
hatch slide guides and other teak trim items with white polymer plastics. I found that the plastics have a shorter lifespan and need to be fastened much more closely. While there are reinforced plastics made for structural applications, the reality is that most consumer plastics, like Starboard, do not seem suitable for things like toerails and grab bars.
Jeff