I feel that the Covenant (Grunts, Elites, Jackals, Hunters) all have a character and soul to them. They have purpose, fear, honor, and all that. Even the Flood had a purpose of always expanding even attacking Covenant if they're in the way. Promethean have none of it. They seem to only exist to attack Humans. And having to always clear areas of them is not fun at all, especially when they appear anywhere.

If they made a squad of Knights fight a horde of Elites, while you try to slither your way to your objective without dieing, then they might be interesting and have depth. That's how I feel at least. Others may feel things about Promethean that I simply don't get.