There is a reason he is weak:
His siege attacks make him the best 'first resort' if your going on expansion-base assaults, as the only things built are usually towers & a townhall with minimal guards.
If your facing the undead or ancients then the aura kicks in and slows them. Enough that even Mortar Teams can escape the Death Knight, even with his aura. Its also hilarious to watch ancients try to chase after you, cause its like they can't even move once in his auras influence.
He is weak for this reason:
He puts emphasis on uprading, his low mana is also heavily influenced by this, as he gains power through:
Studded Leather
Iron Forged Swords
Iron Forged Armor
Storm Hammers
Priest Adept Training
If done right, your base becomes very well made, if you want to make the hero more powerful quite quickly.
Aspect of Rathas is a spell devoted to the original maps Dark God, Rathas. He gave the Battlechest Paladin powerful shadow magicks to defeat the Undead.
Battlechest worked well enough with Shadow Aspect (mana potions needed) and he becomes VERY fast and strong.
Think about in a melee game, if your facing 3 undead teams, you have the obvious advantage in large assaults with your siege warriors. Against live units, you have the disadvantage, unless your siege units can be brought into the base itself. Therein lies the issue, with the Battlechest Paladin and 10 mortar teams in a Night Elf base with little room, they will get killed off very easily. That is unless he has Mare Aura. On a more pressing matter, his slow movement and low attack speed balance the minor things that make his use in combat essential.