This is certainly debatable. Stonehorns are great because 1) They can take and deal loads of damage 2) Be anywhere quickly 3) Count as 10 on objectives (the most crucial aspect of their success). And Mawtribes aren't even close to the top leaderboards in tournaments. Occasionally they eek out a win somewhere, but in most tournament results they fall in the middle.
Kragnos statistically does more damage, but not that much more when you include mount traits, artifacts, and charge bonuses. The difference is only a few wounds. He's also slower, can't benefit from any other spells or effects, and, most importantly, doesn't count as 10. I think you could argue that the Stonehorn is actually much more competitively viable than Krag.
Big numbers on a statline or weapon profile do not necessarily make a strong unit in a game mostly about objectives, synergies, and movement.