I can think of a one case where aluminum or steel would be preferable over CF for a neck reinforcement: a neck that's meant to last a long time (50+ years). CFRP has many wonderful properties, but as with all fiber-reinforced polymer composites, longevity is not one of them. Pretty much anything causes them to degrade over time: vibration, oxidation, moisture, fatigue, chemicals, etc. Degradation affects both strength and stiffness. As environments go, the inside of a neck is a relatively good one for CFRP, except for vibrations and possibly wood tannin leaching. Still, oxidation, moisture, temp cycling, etc will probably degrade it with time, especially if the epoxy and processing are of the bargain variety.
Metals like steel and aluminum also lose strength with fatigue, but more slowly and predictably, and their stiffness remains unchanged. CFRP also creeps more than steel or aluminum, though of course much less than wood. Lightweight and stiff neck reinforcements out of metal could be done -- Modulus * Moment_of_inertia (E*I) is the important quantity for bending stiffness. The lower modulus of alum means a taller section is needed for the same E*I as CFRP. Hollow, C, and I sections have the best moment of inertia per weight. In aluminum, a heat-treated alloy would be best for strength (6061-T6, 2024-T4), but it may be tough to fit the equivalent E*I of, say, 1/4"x3/8" tall CFRP. For higher-tech marketing, 4130 or titanium sheet could be bent into a C. Or mild steel and stainless. Al, Ti, and all steels all have the same ratio of modulus/density; with thin-wall sections, vary the wall thickness to get the ~same E*I and mass.
Bottom line: a 50-year CFRP neck reinforcement is a difficult problem to model, we don't have the field data, and CFRP doesn't have a good track record for longevity. It might last just fine for 100+ years, but there's no way to have confidence. Metals are predictable and reliable.
