Actually i think this comes down to whos power base is stronger and more resilient. Kun or Mandalores. Its one of the most very basic and universal military strategy, your army can be as strong as anything, but it the people behind it that are the key. Could one side destroy the other sides power base. in a war like this, thats what i believe it would come down to. Sith rule by fear and intimidation. Mandalorians on the other hand, actually believe in what they do. i think that Mandalores power base would hold up much better then Kuns.
This would work in any other scenario. However, the Krath and Massassi are fanatically loyal to Kun. When one dies, the others don't care, they just keep going. Not to mention Kun has War Beasts that are bound to his will.