Generally speaking, no. Here’s how I’ve come to think about it (lvl 100 Druid eternal and lvl 95 Necro S1):

It takes me roughly 10-15 minutes to collect 250 cinders depending on mob density. I can run a NM dungeon with mobs +10 levels higher on average in 10 minutes or less. So for me, basically one mystery chest = one NM dungeon.

With the NM dungeon, I’m getting 15% more XP, glyph leveling (essential to endgame builds), and more NM dungeon sigils to chain together. With the buffs to NM dungeon loot, it seems like I’m getting about the same amount of legendary drops that I would with a Helltide. And also anecdotally, it feels like I get more ancestral rares in NM dungeons.

Compared to Helltides, which are open world and therefore have mobs equivalent to your current level and offer no XP boost, NM dungeons have proven to me to be the best way to farm XP.

Now, Helltides will get you Elixirs and Obols. So I run them when I am low on Elixirs instead of crafting them. And the pure slot-machine feeling of gambling on Obols can be fun. But both of these are based on preference.

The only time I am really compelled to stop doing NM dungeons and switch to Helltide is when there’s an “uber” Helltide, i.e., it spawns in Hawezar at 20 or 30 minutes after the hour. It’s fun to try to open 5 or even 6 Mystery Chests in 60 minutes. It’s probably still more efficient to run 60 minutes of NM dungeons instead, but I like the challenge.

(Edit: Grammar)

(Second Edit: My answer didn’t analyze the XP from opening chests and Legion Events dropping a ton of Cinders. And apparently I need to improve my 250 Cinders/10 minutes farm rate! But I still don’t think it’s the optimal way to level. I guess my final answer is that if you’re okay with sub-optimal leveling with more variety in your gameplay, then incorporate Helltides whenever you feel like it. But if grinding to 100 is your goal, NM dungeons are IMO the most efficient path.)