I think the Kemper sounds awesome but on some amp settings it was similar to the GT. In the gain sounds it was better. I have the GT1000 core and I hardly used it for 3 years. The reason I suspect most people don’t like it is that the output filters sound terrible. However if you leave them set to record and turn the IR section off when going into a guitar amp and of course on into a recording platform the improvement is considerable even though it sounds best going into a FRFR or P.A. or DAW. The other issue and the reason it sounds more thin than other modelers is that as with all boss/roland products they never bother to tame the extreme ranges of the frequency range which is necessary to get a more natural sound. If you go into the device and introduce a low cut at a 100hz and a high cut at 4k the sound is abysmally different. You clean amps sit nicely in a mix and you lead sounds come across with warmth and clarity. You can also do this by engaging a parametric eq after the amp block which allows you to cater the cuts specifically to each amp. On some amps a slight push in the mids from either of these eq sections helps them to wake up.
Those who say that it is inappropriate to compare these two together can stop talking. As a user, I only care about the tone, feel, and function. As for the modeling technology, that is the business of product engineers.