I think that the smartest thing do to with *any* gear is to try it firsthand and in-depth.
You can't know something till you really work with it in your own world, on your own time and left to truly both understand what something can do and how to work with it.
I've worked with the KPA box now more than a few times. It has strengths and weaknesses, like ANY gear. Comparing the II to the KPA for me is like comparing a restaurant visit - you can replicate what you cook at home (with your own amp/cab) assuming you have the expertise and knowledge how to properly mic your amp/cab (which you can likewise do with the II, but it is slightly more involved) or with an extensive menu (checking user or premium profiles you must buy) that you can then slightly alter each dish, but must try hundreds of dishes to find what you like and settle on what you like. People mistakenly assume the KPA does a better job at the 'replication' because it is purpose driven to copy things, but that is ignorant of what you can do with shooting your own cabinet IR and then using the Tone Matching capability of the Axe-FX II. It involves a few extra steps, but the results are exceptionally convincing once you've tried it first hand for either platform (KPA or Fractal).
The Fractal II, which is akin to buying the damn restaurant, outfitting it with every prime ingredient, appliance and utensil you could imagine with hardly any sort of constraints whatsoever and creating your own dishes in your own preferred manner, allowing a significantly unique and individual experience and controlling it with ease. Add in the capability to shoot your own Cab IR and use the Tone Matching capability and you have an all-in-one box with significantly deep feature set no matter your intended application.
The Kemper treats everything as one complete system. That allows for a much simpler experience if you get what you want from that singular approach. Regardless of claims to the contrary, attempting to separate the system into components with the KPA IMHO gets a bit dicey. There is an obvious signature to everything it does once you sit down and really focus on the different things it does. Some like that sort of homogenization, some do not.
The Fractal treats everything in a modular manner and creates interactions - with user control over them - down to almost microscopic levels. It does this 'under the surface'; but if you are driven and curious enough, if you want to dive that deep in, you have control over almost all of it. People can tend to get caught up in the depth without realizing that you do not *need* to do so unless you *want* to do so. Once newbies get over the sheer number of options within each block and take advantage of 'effect types' for instance, they realize that the depth isn't perplexing or overwhelming; it is there *should* you choose to use it, not because you *need* to use it. That allows for a very powerful layered and nuanced platform where you can route your signal and control your signal in very practical, logical and useful ways. It is, IMHO, more individual, unique and powerful on multiple levels.
There are so many variables to consider when purchasing gear with all these capabilities and everything needs to be subservient to what you want to do and how you prefer to work. People that fall into 'camps' that must demonize other competing gear never makes sense to me. You are considering very powerful, fully professional level gear with both devices. You have two products that are competing with very different paradigms and similar costs. Weigh what works for you and go that way. I've worked with both because of working with musicians that like the KPA and the Axe-FX II; for me there is nothing the KPA offers that entices me personally or offers any sort of advantage in any way over the Axe-FX II.
To each their own and what's great is that there are choices out there for everyone that wants them.
All 100% IMHO, based on my own first hand experience.