The "hardness" of the wood alone is not the sole factor in its tonal properties or its suitability for applications in instruments. And that of course depends on the actual application. The fiber pattern, density, susceptibilty to cracking, moisture content, etc. all play a part.

As to fiberglass.......Again the actual composition can vary as can the type and cut of fiber used. Then there is the huge selection of graphite composites, some of which can be made very suitable for use. The problem I might suspect here would lie with tradition and not sound. Blind testing might allow a synthetic to win, but attitudes may push it to the bottom of the heap.

Spaw