In no way are Huorns supposed or able to annihilate entire armies, and they do remain on the map for a fair amount of time (whether they are permanent or not, this I can't recall correctly now). Nevertheless, their strength (and utility) lies in the strategic usage of them that one can make, hampering units' movement or creating quite tough barriers around vital elements which should be protected. It's true that the idea was fashioned on the Watcher spell, but that was mainly a matter of animations and implementation; Huorns are naturally nowhere as dynamic as the Watcher in the Water. By the way, I hope the Misty Mountains will retain that spell. It is very much iconic (the motive of the Moria sequence in the first film!), very much impactful in the game (great graphics) and it refers to the obscure lore of Nameless Creatures.