Are exotic speaker cabinet materials overrated?

No, they are not overrated. While there are a few (very few) speaker manufacturers that incorporate cabinet resonance into their speaker design, setting those aside, the difference in sound quality between a speaker with cabinet resonances and an otherwise inert cabinet is huge. Rockport Technologies, Acora, and Kaiser are all examples of this, each using unique materials for their cabinets.

My own Cabinet Design, if adopted, will be something in relation to what Kaiser produce as a Cabinet. 

My material of choice for the structure of the Speaker Cabinet, is the same as Kaiser is using. Even though Kaiser have relabeled it with the name Tank Wood.

The owner of Kaiser has a lifelong Career working with sound, of which a large proportion has been in R&D, funded by Industry with substantial funding. 

Stereophile’s accelerometer tests are quite telling concerning this topic. 

They have measured conventional MDF boxes like the Spendor A7 and Philharmonic BMR monitor that produce barely any resonance. Conversely, some of the special material Wilsons are shown to have at least one high amplitude resonance. 

Based on my subjective experience, I believe a well-braced MDF cabinet, with sufficient wall thickness, is good enough for controlling any otherwise audible resonances. Often times, a speaker’s port resonance will be more audible than any of its cabinet vibrations. A good example is the Q Acoustics concept 50–barely any measured cabinet resonance, but it does have a port resonance at ≈160Hz that I found was surprisingly audible and distracting in my listening. 

It seems like the well-braced aluminum cabinets from the likes of YG and Magico are the most consistently inert in Stereophile’s tests. 
 

I’ve built a lot of speakers and attempt to follow basic guidelines recommended in the North Creek Cabinet Handbook. (Copyright Credit to George Short). The basic box is made out of MDF and Baltic Birch Plywood and is likely sufficient without adding additional layers but I do.

I add 1/4" thick Oak boards on 5 sides primarily for cosmetics but it adds rigidity as well. For the facia I use thicker oak boards, I think for the 15" sub builds I used oak 2" x 4" boards.

MDF and BB have different densities and damping characteristics. MDF is very dense while BB is at least 4 times as stiff as MDF. BB adds stiffness to the bracing and front and back of the enclosure and MDF adds density to all 6 sides. The front and back is a composite panel made of a layer of ¾” MDF laminated to a layer of ¾: BB.

The resonances of the two layers are different and George explains the design advantages and I could only repeat what is explained in the book since the science is beyond me. Some of the design characteristics are more applicable to a full range speaker than a sub-woofer.

There are also recommendations in the cabinet handbook for brace spacing and spacers and cross braces that create a matrixed pattern of “I-beam’s” that adds additional rigidity. The hard wood I add for a cabinet finish just adds to the mass of the enclosure, especially the fascia where the through cut for the driver weakens the front panel.

The surface area of a 1ft3 cabinet is about 20X the surface area of an 8" woofer. Consequently the cabinet only has to move 5% as much as the woofer before it contributes as much 'noise' to the signal as the speaker itself, all of it a distortion of the original, and .5% as much motion of the box relative to the woofer generates unwanted sound, a very audible 10dB below the reference output of the woofer. So, either you live with it, or mitigate it with some combination of bracing, mass, constrained layer damping (KEF), cabinet geometry (Estolon) or materials (Wilson). or use it as a tuning device (Harbeth). Properly done, all can be effective in achieving the designers goals. All are preferable to ignoring the issue. A couple simple braces. A simple diagonal panel to break up both acoustical and panel resonant modes can be a very cost effective solution, adding a viscoelastic layer adds mass and dampens panel resonances, at some cost penalty. From there the sky is the limit, aerospace grade machined aluminum, molded mineral filled resin, concrete, you name it, its been tried. There's no magic bullet, one Holy grail of speaker cabinet building, just solid engineering, testing, and listening.