I would think the flexible pipe would require a fastening method to the skin like the method using fiberglass matting.
Not to be contrary but having built a pedal car using PVC pipe I have some experience.
PVC pipe (Schedule 40) is quite strong. And Schedule 80 more so.
Now there's deciding what method of attaching things. Metal things, wood things, plastic things. Things that move, things that shouldn't move. You are working with a hollow tube with a brittle nature that can result in breakage in an instant.
The weight difference (per foot) between PVC and EMT is noticeable.
1/2" EMT @ 0.29 lbs per foot
1/2" PVC @ 0.16 lbs per foot
Using 1/2" or 3/4" PVC would surely work and can be heated and shaped as required.
If one is to use PVC pipe there is a learning curve in regards to the molecular structure
and how it reacts to heating (time and temperature) and subsequent bending reaction (spring back) prior to setting.
There is the possibility of breaking however if used in a stress point, (AND) you haven't reinforced that stress point.
I reinforced my stress points with wood dowels).
Up goes the weight.
Using plastic (PVC) really depends on the intended use of the vehicle. You should not expect that it will safely hold up under off-road riding conditions. Potholes may have an impact ( no pun intended ) on how well it holds up as well.
Using it
strictly as a skeletal frame for a body may work quite well...IMO and experience.
Was it worth all the effort? Not really, with one exception. I had to learn how to bend and glue PVC together. Now I am left with that knowledge, which I put into making other parts for ... well, whatever.
Not saying don't do it....they said the moon was made of cheese until they brought a chunk of it back.
There are these
https://americanspeedster.com/ and
https://americanspeedster.com/gallery/
Exhaust pipe and carburetor manifold made from PVC.