Just gave a try to all maps on nightmare difficulty, here's my feedback. Maps in the list placed just as in my playthrough order:

Into the Hall of Stolen Faces (hwjam2_yoder): Map theme is beautiful, put poorly executed. Good color palette, bad map layout: there are many neat places that can be accessed by the player through jumps and tricks, but without any secrets so when you jump to some poorly hidden map's deadends you wanna find there at least something. Overall 7/10.

Husks Of The Past (hwjam2_kayne): We DUSK now. Good amount of enemies, bad amount of optimisation. 8/10

When Evil Is Exalted (hwjam2_burnham): Very simple, even basic map. Not very much to say. 6/10

The Trail of Blood (hwjam2_captaincam): Good theme, good secrets placements, good idea with ruined map. Bad pacing, too much walking. Also, during my playthrough some map trigger failed, so I've got to use noclip to find my next destination. Looks like some slipgate or lift didn't work, don't know if map is broken. 5/10

Oblivion (hwjam2_artistical): Very good horror idea, but can be broken really easy by not going in the episodes natural order, also by jumping through gaps to the final slipgate too early. In both cases you can softlock yourself, ruin your playthrough. And the "shoot to light your way through" gimmick isn't really good. So 8/10

In The Walls Of The Ziggurath (hwjam2_chrisholden): Very satisfying map. Good design, good amount of enemies and arsenal, awesome art direction. One of the best in the entire pack. 10/10

Misadventure Galley (hwjam2_greenwood): Art stuff is just gorgeous, pirate theme is kinda rare in quake. I could use some more breakable props in the lower decks to make more space and probably more early access to quad and other goods. 9/10

The Black Reincarnation (hwjam2_jcr): Map theme is great. Good exploration, good secrets placement. Good idea with walkable webs, good idea with using wizard as zombie scorcher. One of the best maps overall. 10/10

Candles of the Damned (hwjam2_comradebeep): So this is the e1m2 repack, but very cool. Underwater exploration is great, and the secrets placement insired by ad_sepulcher. 9 backshooting gargoyles out of 10

The School Of Dark Magic (hwjam2_pinchy): Art direction with overall white stone & wood theme is good, nonlinear exploration is good. Lack of ammo is bad and makes enemies placement/ambushes even more frustrating than they already are. Map layout is bad, all these chandeliers and other stuff on the floor aren't making fights easier, especially these which damage player. 6/10

The Worship Condensate (hwjam2_chedap): Map layout is awesome, colors and the balance between dark and pitch black is excellent, traps and arenas are very good. Did I said that level's art direction is magnificent? 9/10

Outhouse (hwjam2_maxx): Map idea is pretty cool, nothing gives you more horror as destroyed toilet in the woods, art direction is great. Ammo is very scarce, to the point of being more bad than good. This makes it pretty hardcore in the medieval section. Otherwise cool level so 8/10

Trick or Treat (hwjam2_thulsa): This one is a dissappointment. I've collected dogs book, vore's book, wooden planc book, wooden crates' book, silver key and the trap ceiling's book, and the lava platforming book, and I STILL can't finish level despite having 3/3 secrets and 21/22 kills. Lots of missed oportunities for secrets and interconnections between puzzles. 3/10

Honey I Shrank The Ranger! (hwjam2_here): What a ride! Good amount of fighting and jumping, whole arsenal of weapons, map theme and level layout are the top notch. Just one note, if the knights at the end fall into the lift shaft it makes impossible to finish level without killing them. Also it could be even more fun with less invisible walls here and there. So that's 9/10 

Spud (#38): hint about the merciful door: the book at the beginning tells you that "He will judge the living **and the dead**". 

But all the same, now that I've unravelled most of the map (all kills, 9/10 secrets) and seen quite a few options for shortcuts, it seems like speedrunners could have a blast with this map. I haven't tried it, but it doesn't seem hard to bypass the gold key as well as the whole area containing the "merciful door", and go directly for the silver key.