This honestly just does not feel true. Obviously I am just speaking from my experience but NM dungeons generally give me 1 Legendary aspect (maybe 2 or 3 if I am super lucky. I guess about 40% of the loot is ancestral so that is decent. An hour of Helltide in the same amount of time gives me like 6-15 Legendary aspects and an assortment of Ancestral gear as well. Plus Forgotten Souls which are obviously useful. Plus Helltide is just more pleasant to do.
Maybe I just don’t do NM dungeons fast enough.
Kind of weird considering I get tons of ancestral gear all over the place outside of Nightmare dungeons. Not to mention legendaries as well.
The only thing Nightmare is good for is the experience and leveling your glyph. Apparently also if you can do super high level you can get a lot of 800+ ip loot as well.