Regarding FRFR it is a concept that makes sense when you want a device that plays anything that you feed in the most linear manner possible.
But so many people here in the forum have spoken about a somewhat underwhelming feeling about this kind of solutions. Because we don't want a linear speaker. We want a guitar speaker. And a guitar speaker is not linear, ever.
I have a different opinion/understanding to this: "We want a guitar speaker. And a guitar speaker is not linear, ever"
I think this misses the point of the need of an FRFR speaker completely. When you use a linear speaker it is not to flatten the response of a guitar speaker you want to reproduce (not sure if this what you meant t say or not). In contrast, if you have a guitar speaker A and capture its IR with a Microphone "M", you will end up with a sound sound "AM", the need of the linear speaker "L" is in order to then get a faithful reproduction of "AM" and not a colored sound "AML". In other words the linear speaker is used with the aim to un-affect the original signal.
One problem with the Mic variable and one of the reasons why a speaker/cab reproduction never feels like the original speaker (apart from the color it introduces), would seems the introduction of more frequencies that were not present in the original signal coming form the guitar speaker. Hence the existence of PureCab and GRFR and other DSP tricks. CK is trying to address this with the speaker imprints.
The other problem that CK is trying to address with the Kemper Kabinet is the dispersion of the signal and the beam effect. The Beam effect is what everyone mostly attributes to the Amp in the room effect whereas due the position on our ears in respect the signal beam, make us hear a different sound. Most FRFR speaker I believe have greater dispersion thus making us hear more frequencies than we really want.
In conclusion. I do not think the "linearity" of a speaker is the problem at all, but instead other physical effects as per above.