Premature juggernaut charges
Juggernauts almost always waste their charge ability before seeing the character. It's not uncommon to see one on the opposite side of the level with monster detection and then hear a "rolling thunder erupt" just a few turns later. Either they should wait until they can see the character, or the ability should recharge over time - as it is, you almost never see their charge in action, and they end up functioning as off-brand molochs.