Humans can use full body armor, meaning they take less health damage proportionally over the course of a battle, also, headgear makes them extremely resistant to being taken out by headshots, the single largest weakness of the skeleton.
To be clear, I am reporting this mostly based on other players who have reported on their exploration of the combat system, but also based on my skeleton vs. Holy nation playthrough, where skeletons with eighties across the board in their combat stats were being disabled (temporarily) by human opponents in vastly inferior gear with lower stats. We won these battles, but by the end of most I had half or more of my skeletons effectively out of the fight due to blunt damage or wear. The assault on the capital was especially bad, I almost lost that one twice with an army of more than thirty skeletons in primarily masterwork gear, Mk III weapons, and combat stats above eighty. Humans are stronger combatants than skeletons, period.
While Skeletons WILL eventually wear down opponents, so long as they have adequate repair kits, however they get knocked down much, much more frequently than humans or shek. Meaning they spend less time damaging enemies, and more time healing.
Further, even Beep can in fact become the strongest warrior on the continent if you grind him up high enough, but the base power disparity is definitely present between the races, and is clearly reflected in the way they are presented in-game.