The advantage that GaNFETs have is they have very low parasitic inductance. What this means is its easier to build a module that is low noise- and by noise, I mean the kind that radiates from the module over the air and thru the AC power. Its important to keep switching noise and parasitic noise low because it can mess with other gear and affect its performance. As an example, our class D amp makes less noise on the AC line than most tube amps. In terms of noise that it makes in the loudspeaker itself, 90% of that comes from the input buffer circuit, but even then is obviously quieter than tube amplifiers (I use horns at home FWIW).
We've been docked a bit on account of the cost of our class D (which is cheaper than most of our tube products). We built it to be rugged (so it will ship without damage), to last decades, to be easily repaired and updated and to that end it has a fairly expensive chassis and power supply. It would actually be more expensive if we used someone else's module.