Hi,
rigidity is the key design/build target of any machine. I see no reason why GRP cannot be used to advantage.
Typically most GRP projects are designed to maximise strength.......but strength is not the same as stiffness. Before committing a lot of time and materials to build
a design use an FEA program to determine the rigidity or lack thereof.
The only weak point I see is those round rails. They are about as rigid as soggy spaghetti.
Craig