Every thing in the life has two faces advantage and disadvantage. Using fiber glass in the cars body reduces its weight and nowadays fiber glass is enforced with carbon fibers which give it more strength and toughness.
Metal body surely increases the dead weight of the vehicle hence more fuel consumption/ Km. Apart from this, the consequent effects like increased drag force, eddy currents demanding high capacity engine. With Fiber body all these effects could be mitigated. However, the risk is high even in the event of a moderate accident.
These fibers'bodies are enforced with carbon nanotubes which make it harder than steel. It is very safe. However, all these precautions don't delay the vate, as you know.