As spotted by Gamesradar, a particularly stubborn mathemagician has rolled up their sleeves to properly prove whether it's even worth it to run high-tier nightmare dungeons in Diablo 4.
Posted last week on the game's subreddit, fpsthirty put in a staggering amount of work to confirm their suspicions after tiring of the level 100 grind, wanting to "combine farming with an activity that is interesting and useful to the community … I decided to complete five dungeons of each tier, so that by the end of the experiment I would definitely [reach level 100] (I regretted the idea already on [tier 30], but I brought the plan to its end)."
This staggering one-gamer survey saw them complete over 400 nightmare dungeons in 18 days, cataloguing a staggering 8,104 Ancestral rarity items. Turns out, the tier doesn't really make much of a difference to raw item power—the top 3 grabs came from tiers 21 to 100, which is a massive variance.
Fpsthirty's helpful TL:DR lays out the core issues. From tiers 40 to 90 the loot's almost identical, with a piddly variance of 3 points across 70% of items. "In [tiers 91-100] the lower bar of item power has increased by 2-5 points, but the percentage of [item power] 785+ items is the same as on [tier 40]."
The rate of rare drops is increased the higher you climb, with tier 100 giving a 60% higher Ancestral drop rate when compared to tier 40. But if the item level variance isn't that big, this might not even be worth it as commenter Actual__Wizard explains: "If you want the maximum chance per run, do [tier 100 dungeons] … realistically you want max chances per hour and [tier 100 dungeons] slow most characters down a ton, so you're going to have to figure out what tier makes the most sense for your character."
This tracks. It doesn't matter if you're more likely to get an Ancestral item if the dungeon takes you way longer—there's a sweet spot between completion time and nightmare tier for every character. Needless to say this is super counter-intuitive when the game's systems point to tier 100 as some aspirational milestone. Why bother climbing if you've more chances to get something nearly as good at a lower difficulty level?
Ultimately, the consensus seems to be: do the highest tier you can until you feel slowed down, then grind that. The power boost from later tiers isn't worth the investment and pain, especially if you're still grinding to max level.
The full thread has plenty of other advice from the numerical wunderkinds of the Diablo 4 community, such as DownhillYardSale's collection of over 11,000 runs which points toward Uldur's Cave as the best spot for item drops.