I've been playing around with Juggernauts and have come to a fairly decent solution for balancing these guys out, so I'll just toss this out as a suggestion to improving these guys:
1. Remove the 25% Splash Damage from base Juggernaut ability (this is mainly to help the AI because they can't handle the tactical positioning voodoo that is needed to make the most use of it). Reduce cost to 250 (from 400).
2. Move the 25% Splash Damage into Uncontrolled Rage, making it 75% in total, this improves the trait to make it more worthwhile - primarily for players, encouraging more design diversity. Increase cost to 60 (instead of 16).
3. Lower the damage bonus of Brutal Nature trait to 50% damage bonus, this trait is just too much of a no-brainer at 100% bonus. Increase cost to 60 (instead of 16).
4. Reduce the HP bonus of Ignore Pain trait to 30, but move the 10 HP into the base Juggernaut unit, this is to lessen the effectiveness of the trait so as to give other traits a chance. Increase cost to 90 (instead of 35).
5. Add a -10 base Accuracy penalty to Maul trait, it's too easy to get high accuracy with Fortress upgrades. Increase cost to 60 (instead of 0).
6. Add a +1 Movement bonus to Frenzy trait, this helps Juggernauts chase down kiting archers. Increase cost to 60 (instead of 10).
7. Add a new trait "Harvester", that allows the Juggernaut to gain +5 HP whenever an enemy is killed. Costs 60.
The end result is a slightly nerfed base Juggernaut unit, 50% less atk (Brutal nature nerf), 10 less accuracy (Maul nerf) and no inherent splash (again, to help the AI) and costs a bit more (trait cost tweaks - 460 instead of 451, not including weapons/accessories). However, all the other Juggernaut traits are buffed up and given use (except for Heart of Stone - which is still a bit useless, maybe add 5 def for the other damage types?). This allows for more diverse Juggernaut designs (like a Brutal Frenzy Harvester...) to change up the core unit a bit. The cost change of the traits also introduces the possibility of using cheaper "base" Juggernauts without the extra traits.
Just my 2cents...