Most of the "propriety" plastics are fiberglass reinforced nylon. A close look at the new "gray" Walther will show the pattern, just like a truck tool box.
The proportion of glass to nylon changes the characteristics a lot. Some knife makers use a higher percentage of nylon, which give the resulting grips an "oily", more flexible feel. It's perceived as less expensive than high fiberglass, which has a dry, rigid characteristic, and can be almost indistiguishable from G10 fiberglass laminate.
Using +P+ reloads in a .380 LCP will crack the frame, as one poster on a website reported, with photos. Another long infamous post is a pic of a Glock which a German Shepherd used as an impromptu chew toy.
Plastic does very well for what it is. Any material can be abused beyond its design spec, like the steel slides on service Berettas that failed due to submachine gun ammo being used.