The simple answer to why Mill decks aren't going to be AAA material is because of
Rotom UD. The reason Durant was successful in the first place was because it didn't have to worry about prizing a Durant or two thanks to being able to swap out any unlucky Prizes. Without it, every Mill deck in the format is controlled by luck, and sicne you didn't run any GOOD attackers in the deck, you had no way of fishing out any of your Prizes. If Rotom comes back as a pseudo-reprint like Unown from Ancient Origins, it could certainly make Ninjask/Bunnelby at least decent to use, but even if it did, many of the Standard/Expanded mill decks have their own problems...
-Durant NVI has pathetic HP that can easily be donked, even if you have a Shield Energy and a Hard Charm equipped. Also, if it ever did come back, Heatmor DEX would be the penultimate nail in the coffin for it.
-Aggron DRX / Sableye DEX is arguably the best one to use in Expanded, but it relies too heavily on Items like Devolution Spray and Trick Shovel, so any lock deck at all would make it nearly impossible to play, and with so many Item lockers like Trevenant XY, Seismitoad-EX, Zebstrika NXD, and Vileplume ANC, it's too risky to play competitively.
-Swoobat BCR would be hilarious to play with Gardevoir NXD and Ancient Togekiss combined. In fact, I'd say it's crazy enough to work, but the fact that it needs 2 Stage 2 lines to work means that pressure decks like Landorus-EX / Crobat will shut it down before it can do anything.
-Rhydon XY may be able to mill the most cards at a time when combined with Ancient Rhyperior & Celebi-EX to keep it sturdy, but you can't expect 2 coin flips to work successfully. That's a 25% chance of working, and if you are allowed to reflip with Victini NVI, that's still only around a 37.5% chance. (estimate)
-Ancient Bunnelby may be good for milling 2 at a time, but that's all it does. Every other mill card I've mentioned so far mills (1) more cards than him or (2) mills based on a seperate factor. Durant maxes out at 4, Aggron mills 3 at a time and more with several Devolution Sprays per turn, Swoobat is infinite based on your total amount of Energy, and even Rhydon of all things can do around 8-9 with a few lucky flips.
-Ninjask ROS is very consistent, but the low HP and factor that restricts you to being Active means you need to run tons of Switch and Hard Charm in your deck, reducing your consistency severely.
Still, even with those problems, I could find quite a fun experience with them. I've tested Ninjask/Bunnelby a few times with average results, but the other ones could be great fun to use.