Something in Legion has been bothering me for a while, it's why the Invasions and Broken Shore and Argus really irk me, and I think I found out what it is.
Demons in Legion(specifically the non-rares) are just the most pointless enemy in the game. From a game design perspective. Especially on the Broken Shore and Argus.
Firstly, they are always somewhere between a boss mob and an elite in terms of health and damage.
Secondly, they drop nothing at all of value, compared with Timeless Isle mobs that dropped coins, Isle of Thunder keys and other stuff, Tanaan that dropped crystals and other stuff. Like they 99.9999% of the time have nothing of value or interesting on them. Only rares or boss mobs for quests seem to carry even a green with any amount of regularity, although I've got nothing from invasion bosses probably 50% of the time outside a trash item.
Thirdly, you don't get any reputation with Legionfall, the Army of the Light or any other faction for killing them outside of specific world quests.
There is no incentive to kill them or fight them at all. You actively don't want to fight them, because of such a waste of time it is. I've seen people pull teams of demons into other groups, just to get rid of them, because they are so utterly unrewarding in any way to fight, that they are below the level of raid trash. Stun them, root them, daze them, sap them, vanish, just anything to get away from having to spend time fighting them for nothing. They are like 300% stronger than a regular mob, but drop 300% less stuff, and don't even give rep. So whats the point?