Finished, at 54h ingame time (and a good chunk more of unsaved time), 345 secrets, and a score of 37103 on hard/nightmare. Overall, probably my favorite dungeon crawler ever. Some random thoughts, spoilered for size but also for mild spoilers.
The star of the show is still Adrageron's level and encounter design, so much variety for his 3rd fullsize mod in the same engine. Many levels manage to have interesting gimmicks to make them stand out, and there are some extremely cool features to discover that affect exploration.
With so many secrets there are plenty that are just a hidden button, but there are a lot of weird neat puzzles. A big puzzle level was a particular highlight for me, though some might find it a bit overwhelming as a single chunk of puzzles. Pretty much all secrets are also nicely signposted, there's a lot less of a problem I had in The Guardians, where the secret itself might be nicely hinted at, but the answer was a single note hidden in some grass clipping through the ground on the other side of the map.
The endgame combat encounters are generally significantly tougher on average than The Guardians, which in part might be because of whatever nightmare does, but in part is because of the aggressive new enemies and arena design. It really felt like I got some use out of the power you get from enchantment etc.
I'd definitely recommend a caster of each school; the extra equipment juggling from having 3 casters in the party adds a lot to manage during combat, which is fun, and each school has some extremely useful spells which would be quite painful to play without. The replacement items are there, but definitely too expensive to use as often as you'll cast the spells. With all the reworks to everything else, damage magic is still completely worthless outside firebursting mosquitos and some similar enemies. The best use of fireball is to light up dark passages.
There's also a lot of skillpoints and xp to go around, which helps spread out those casting skills. I ended up with 2 level 20 characters (that's 1M xp) for the endgame; the tough gains modifier to reduce xp should be quite managable. Respecs are also very available, which came in very handy as the requirements on your party change a lot throughout the mod.
The two downgrades from The Guardians imo are the economy and basebuilding. In the Guardians, the materials were always rewards for exploration, and you often had a lot of options to spend them on and had to choose what to take at any given moment. Here, it feels like at least half if not more of your materials have to come from just buying them from a randomly restocking store. And a lot of money comes from selling random junk. So there's both fewer choices to make and its less tied in to exploration. I ended up not building some epic items because I hadn't bought up enough ore before I needed it when I saw it, so would just have to wait a week and hope the store restocks with some.
And the base is just a lot less cool than The Guardians Livingrock; a lot of the more useful features are now spread out over the map, and instead the base itself has some goofy but mostly useless farms.
The plant box is neat, though. Significantly weaker than an alchemist (which is good), but more versatile.
A more spoilery list of the things I missed:
1 epic item/scroll. Maybe a bow?
The Grove buried treasure chest.
The Hammerhills buried treasure.
1 secret each in Burrows, Outkeep, Deeps, Necropolis (the pressure plate is visible through a grate next to an altar) and Seven Tombs (presumably the guy who wants to see some fireflies).