Haven't heard these in particular, but I have used a bunch of transducers that turn solid panels into speakers (Clark Synthesis/Aurasound/multiple Dayton transducers, and the bass-specific IBeams and Buttkickers), and they all have the same issue.

The material you stick it to has it's own absorption/reflection/resonance properties, which become audible when you use it as a speaker. You can easily tell if it's attached to metal, plastic, wood, or glass. It doesn't sound "like the panel is a speaker", it sounds like "a speaker behind the panel".

For bass, thicker/stiffer materials (or those with less surface area) need more power, which means it effectively reduces the output when used with the same power. Same transducer, same signal, will have wildly different response curves if attached to a side window vs a rear window, or a window vs the dashboard.)

For highs, basically everything absorbs high frequency sound (that's why you tend to only hear low frequency bass outside a loud car or club). No matter what, the high end is neutered, they all lack the "sparkle".