Devourers are prone to CC, devourers are limited to 6 per deck, devourers require quantum to be put up, and devourers can only give quanta if your opponent has some to steal.
4 Things that make it entirely different from a pillar/tower. Towers are rarely target of destruction except for spells like Earthquake whilst devourer is a favored target. Any mono-darkness with less than 5 pillars/towers/pendulums is as good as worthless in my opinion.