I've used AX FX for my band's first EP (Ring Hollow) and my Vocalist is a up and coming producer (he uses Helix for his projects currently), and lastly, I have owned a Kemper since 2016.
The only way I can describe the subtle differences is "authenticity" the kemper has exactly that. What I have noticed is a lot of people who are exposed to AX or Helix are usually sold on those units and believe it is the same as a Kemper.
Modelers VS a profiler...its hard to compare. I have heard tones from each source and appreciate all of them for what they are. I think you hit it right on the head with this. Because it does require more patience and an ear for authenticity using a Kemper..some people just don't have the patience.
The other systems are perhaps more immediate, have more evolved characteristics, especially in the effects / editor, but their sound seems more distant from the classic analog sounds of the valves.