The major ambition is easy, but unstoppable juggernaut is as much a detriment as a bonus, since it prevents units from ever being able to stand in cover for their own protection and has to be applied to all units in the stack, even ranged units. Furthermore, the minor renown reward is basically asking you to waste 10 turns with this hero personally excavating when that's a job you could have a scout or a T1 doing.
Maybe make the army passive only apply to melee units? And increase the renown from excavating, or change it to something more appropriate for a heroes' time, like winning underground battles or something.