The FRP panels I've seen, while more rigid than the plastics, are not all that heavier in construction or strength.
I'd never trust one to walk on it without protection and support.
I'm including them in this category of "plastic" roofing.
I do not think you'll ever see a plastic roof on an actual building, such as a house.
They are almost always found on freestanding and attached type roofs covering lanais/patios/porches and such.
We have 3 plastic roofs, the back lanai attached to the house, a free standing front lanai, and a freestanding pergola covering a spa tub, all with flat or shed roofs.
I've also seen them installed on fairly large temporary storage/shop/etc type buildings with a normal gable type roof and visqueen covered walls.
Plastic roof panels do not have the durability of a standard type roof.
I believe they generally project a 20 year lifetime.
Our 3 roofs were not properly installed and lasted only about 10 to 15 years.
The pergola's roof has already been replaced.
The front lanai is scheduled for the future after a few other projects are completed.