Composite do not fair well with compression load, the fiber are not being stressed, and the epoxy matrix is relatively soft, so the actual rail will creep under load, and your accessory will eventually got loose and fall off or the barrrel nut will eventually come loose. That is the reason why most composite rail system uses metal rail and barrel nut.
Other than that, most fiberglass, garphite, and.or kevlar based composite should be able to withstand the heat generated by an AR, basically use a high temp epoxy as the matrix will work.
The typical FF rail I saw is what the aero industry called "black aluminum", which basically replace the metal with carbon composite, gain some weight advantage, but not utilizing the most out of the material. To gain maximum strength and mechanical stiffness off a tube, the best way to make the tube is filiment or tape wounding, and the worse way is to use chopped strand fiber. If holes are needed, it is best formed during wounding, because drilling holes will result in broken fiber, thus loosing strength an d stiffness.
One dis-advantage of composite is the thermal conductivity is lower than aluminum, so the weapon will cool down faster.
Filiment or tape wound tubes are expensive to manufacturer, but one can purahcsed already wound tubes from Mac-Master Carr, and bound in and shear tie the require metallic components.
Be very careful of mixing aluminum with graphite. They will generate galvinatic reaction, and the graphite will corrode away. I seen numerious products that mixes aluminum with graphite, and sometimes I wonder if those people know what they are doing. Best material to use around carbon is steel or stainless steel.