The Acid Reaver is a great unit killer who is meant to get into the fray, fighting to the death with his powerful close-ranged spells. Skills like Caustic Breath and Acid Sheath allow for his greater survival. He is great for base-sieging, able to reduce the armor of enemy units and structures in his area, allowing buildings to be destroyed in an easier fashion.
It must be taken into account, however, that the Acid Reaver is not the most mobile hero. This may be a great liability to some players, but this inherent weakness can be made up for with his powerful skills.