As someone who doesn't know the Kemper as well as say a unit like the Axe, the question does arise to me at least, of how IRs are differentiated from a profile, as a mic is picking up a full signal chain, and how the taper of knobs on the kemper compare to the real unit.
From player point of view.
I don't think cabs are IRs (you can convert IRs to cabs tho). Other than that, I don't really know, it's sort of sorcery (and probably patented), but it works well, it really does. You would have to try for yourself.
Knobs don't work like on real amp counterpart. It's more like having a very good generic tone stack/EQ. Except gain, which, well just adds gain and have huge range. Is this good? Yes and no, for some amps it works great, for some (quite rare) it doesn't, mostly because that particular amp has (in real version) sophisticated tone stack which affects feel, drive etc. Now, if we would look at it in negative way, some may say that most tube amps have this "sophisticated" tone stack. But, as I said before, personally I don't think it's the case. I have a couple profiles of Marshalls and Fenders which are profiled at 30+ settings and I could (like poster above me) delete most of them because tone stack would get me there without problems.
Tweakability is there if you want it and it can alter amp sound in very radical way (there is tube bias, clarity, pick attack, etc.. have a look in the manual). The difference between Kemper and Axe is that you start from pre-baked amp model (axe) or profiled amp sound (Kemper). My opinion is that it's easier and quicker to start from a profiled amp sound, because in many cases there will be no tweaks needed (I am saying this as a player, I am not a tweaker) and you can always load up different "amp model" - a profile. Having said that, many Axe presets are just right as well.
I suppose all these debates what is better come from people who feel the need to justify their purchases and cork sniffers who can't play and just mess around with gear endlessly :). In reality, both units are great and top of the game.