Are carbon fiber speaker cabinets inherently better than wood or metal ones

I’m guess here. The speaker designer is making compromises throughout the design process and view the various designs as a system. 
 

I imagine Sonus faber believes using natural materials for the cabinet is a critical aspect of their design. An I think Focal, Wilson and others would make the same comment. Not sure what Magnapan’s perspective would be though!

Ultimately it there’s excellent options and beauty is in the ear of the beholder 
 

Carbon Fiber is an expensive material and very time-consuming to do properly. And very difficult to do at scale. If you want to see what's involved in making CF parts, I recommend spending some time on the Easy Composites YouTube channel

How many violins or highly regarded guitars are made out of carbon fiber?  Pianos?  Just wondering.

All things resonate carbon fiber does so and it sounds plastic-like. Wood resonates and if done right can be low in level and even in frequency this sound good and doesn't detract. Alunimun is a cheap material marketed as a luxury item in audio because audiophiles don't remember that it's used in beer cans.

Sorry, johnk, but this is wrong:

Alunimun is a cheap material marketed as a luxury item in audio because audiophiles don't remember that it's used in beer cans.

The machined aluminum cabinets by Magico and Rockport (for example) are both complex and very vexpensive to make. No one applies more engineering to their speakers than those two companies, imo (there are others on par I assume). 

Rockport has combined a carbon fiber exterior shell with an aluminum interior housing for their newest speaker, which saves some money. Their top of the line speaker uses aluminum interior/exterior shells. Magico is all aluminum.