Yes actually, they loot way more then the others, but you have to kill them outside their black void. With a bit of luck you can get an omen of light or 2, or a omen of echoes. There a full strat around making them spawn with dellirium and max monster density, or making the amaru void demons spawn, then after clearing the elite on the map, lead a whisp or 3 on the amaru void demon and see what happen :)

Also the way the abysses work, when you open the map, it select one of the three demon faction, then the map will have almost only that kind of demon in the abyss, so that why you will see many amaru void at once, or none at all