[instarx]

I used to work in the health and safety area of the advanced composite industry and here are my thought as to why composites are not used much in cars.

1. Expense. The materials are expensive on their own, but the biggest expense is labor to make them. These parts have to be made by hand, alternating sheets of partially cured composite materials at different angles. Body parts would have to be hand-made. Roofs are easy because they are essentially flat, but fenders etc, would be extremely expensive to make. Anything with an internal structure like a door would be outrageously expensive.
2. Environmental. Although fairly inert after they are made, composite parts are not environmentally friendly to make. Just the resins alone are hazardous when liquid, and the amount of hazardous vapor produced during curing is considerable. Each part has to be cured under heat and vacuum for DAYS, so it is very energy intensive.
3. On severe impact these materials can fracture into some long, thin fragments about pencil length, but they are light-weight and bendy so would be unlikely to cause injury. Composite panels are actually quite strong when impacted and are not likely to shatter into large pieces that fly around. Helicopters use composite armor, so it's not like it shatters easily.
4. In case of a fire, everything electric in the immediate area might be shorted out by the smoke (composites will burn). The smoke has fine carbon fibers in it which short out electronics. When I was doing that work about 20 years ago investigators had to enclose their radios in plastic bags to keep them from shorting out when they investigated military plane crashes involving composites. Don't know if they have worked that problem out yet or not. There may even have been an electrical sub-station that was shorted out by smoke from a composite fire, but that was just a story that was making its rounds in the industry and I did not see anything published about it (not surprisingly).
5. Health concerns. When sanded, very small fibers are released into the air that may be a serious health risk, like asbestos. That's something of an unknown, so it is probably best not to put too many composite cars in the hands of local body-shops.