I have an Everest and have not tried the Karnali.
The edges on the Everest have been lifted higher up than on the Burn series, other than that the Everest has a touch more length and more volume in the bow and quite a bit more in the stern.
I have used mine of some decent class 4 creeks and rapids with drops and long continuous sections....I LOVE it!!! One of the best boats I have ever paddled. It is stable, fast and turns easily for its size, especially when driven on its edge, as it should be! And its big!!! which i like! Did I say its fast....
However the plastic does seem a bit soft for a boat designed for creeks and hard use. Mine is developing a bit of a 'pregnant belly' under my seat (because I am a fat bastard??).
I like the edge and am not interested in edgeless round boats which I feel only really excel if you run super-steep lower volume class 5, which I dont (eg the Jefe which many guys like and many hate because of its banana like shape).
The Karnali does look great too though. Perhaps look at a Solo and Magnum. The Solo is awesome but I would ideally like it to have a few more gallons of volume for my height and weight (perhaps we will see an XL in the future?). It also just won a boat of the year award in France and deservedly I might add. The Solo also has hands-down the best safety/step-out and rigidity system going, by a LONG way.
My favourite creekers/runners are the 3 Pyrnanhas (burn/everest/karnali) and the solo.....