About Ancient Ent as Tank, low mobility is a burden I agree, low initiative is a handicap I agree too. But let see it from a defense point of view with few sample math:
- First quote, the life ratio: Life per 100 Leadership is 83.33. That is huge for a level 5 unit and that match or does better than most level 2 to 4 units and many level 1 units. For example Horsemen has a lower base life ratio of 72.22. Well for a warrior with Iron fist skill the picture is different but for mage and paladin that cannot be improved without a very very rare items set (Knight set). Such high Leadership cost means an average Leadership lost of 500, that's significant but the more Leadership you have the less 500 is significant. Still, Ancient Ent have an exceptional life ratio for a level 5 unit.
- Second quote, the defense. For Ancient Ent this brings between 50% and 25% less damages than Horsemen. See bellow the little math for explanation.
- More over Ancient Ent also have 50% resistance to poison and 10% to physical. For example in comparison Horsemen has 20% to physical and 20% to fire.
So if you consider the Divine Armor trick that fully cancel Ancient Ent penalty against fire, the 50% to 25% less damages than Horsemen, the excellent life ratio, it's clear that Ancient Ent is an exceptional defender, good for a tank, could worth the low mobility and low initiative in many battle. For a mage I'm almost sure but haven't yet test. For a Paladin more tests need to be done. For a warrior that could be a less interesting option with an investment in Iron Fist. Still damage reduction is more important than life ratio and even with warrior, Ancient Ent worth a try as tank.
About the reduced damages:
Higher units compensate lower leadership ratios with higher defense and attack. For a Tank let see the defense of Ancient Ent: 50. In comparison a Horsemen has 25. For an attacker with an attack higher than enemy defense, each added attack-defense point gives 3% more damages. But let consider the less favorable option, as the hero has many defense bonus and with a such high defense base the average ratio will be more roughly 2% in average. That means roughly 50% less damages than Horsemen. If we use a negative hypothesis with an average of 1% instead of 2%, it's still 25% less damages, that is huge. That's why Ancient Ent are so strong, exceptional life ratio and very high defense.
EDIT: As already quoted items can change many things, for example the Steam Armor inflict a huge mobility penalty but if for some reason you still want try it, Knight become non mobile as Ancient Ent but gain 30% defense ie get a defense of roughly 36 but also has 30% resistance to physical and fire. With Iron fist skill, circle attack and 30% attack bonus given by Steam Armor that can build an interesting non mobile tank option.