Stage 12-3 nightmare gives about 10% more xp per energy than brutal, albeit less silver.

it also gives considerably less mystery shards

Which may or may not matter, to you. I don't run campaign at all, beyond just for the quests, wherein I run the easiest difficulty so I don't waste more energy than I need to. But mystery shards are entirely irrelevant to me as well.

I find your last sentence surprising... 

*shrug*. I use three daily + 10 more every five days, which I get more than enough of from scalping the market. I have no patience to use greens for fusion events, so they tend to just stack up.

Same.

I can think of at least 2 fusions where having a huge stockpile (4K+) of Mystery shards carried me through the champion chase event or allowed me to power level champions for a champion training event when I was short on brews. Also good to just use as leveling food when I want to save some energy. 

I buy every one I see in the shop and actually get excited when I see 3+ pop up at once.

I don't think I've done a fusion in the last 3-4 months where I haven't opened 1k+ mysteries, another reason I do actually value doing Brutal 12-3 and 12-6 from time to time (over the dungeon farming)

im just saying the rewards for Nightmare should be at least 25% superior to those of Brutal, should be more than twice the experience to help with wasted surplus experience, should give at least same rewards on everything with a little higher drop rates on maybe 3 star champs, maybe ancient shards etc...

I think, for myself at least, that Brutal would still be more efficient for leveling food even if the rewards in
Nightmare were increased due to the time it takes to complete a 12-3 run.  

I can currently complete a 12-3 Brutal run with 3 food champions in around 13 seconds 100% of the time. I can't even complete 12-3 Nightmare consistently with my farmer & 3 chickens. 

Honestly, if I were at a point where I could consistently farm 12-3 Nightmare in less than 30 seconds with a farmer and 3 chickens I wouldn't bother farming in campaign anyway.  

For a long time, I did Nightmare 12-3 with Ignatius as my solo farmer. Runs would take about a minute, vs about 7 seconds on Brutal 12-3, which is less time-efficient sure, but the payoff of energy saved was worth it, to me.

You practically *never* run out of need for food, so until you get a good solo farmer for dungeons, you'll need to continue running campaign.

I honestly don't think anyone should ever farm NM. I can't think of one player cohort (f2p, p2p, end game, early game,   ...) where it is optimal. 

That said, I do plenty of sub optimal things far too often myself.  This will never be one of them.