QuakeSpasm doesn't have any of the Nehahra-specific engine features, so it's best not to use it for Nehahra. The maps load but various stuff will be broken (I'm not sure to what extent).

Zerstorer on the other hand, I think runs on any Quake engine (I remember playing it on Winquake when I didn't have a gpu.) 

I just did c:/quake/quakespasm.exe -game nehahra -nehahra -->Single Player->New Game and it did a Quake Error "sprites/explode1.spr" has wrong version number, should be 32".

Plus Nehahra requires some qc functions that aren't in original Quake like PF_changepitch, PF_sqrt.

(Also the progs.dat with just certain maps like to crash Quakespasm if you remove the sprite32s. Quakespasm Spiked ironically has a fix for that.)

For completeness, the wrong menu shows (it shows, I guess the "Seal of Nehahra" menu instead of the game menu --- Nehahra's actual menu is named differently), the fmod .xm music won't play, Nehahra needs some extra cvars available or it won't work right (which I think is limited to cutscenes). Plus Nehahra does not use what we view as the normal "fog" worldspawn entity, it rolls it own way via an svc. Nehahra also has some custom effects (certain particle numbers it treats differently).

Even with a non-Mark V engine that properly supports Nehahra --- all kinds of stuff likes to mess with your settings during the course of gameplay. You might have a certain fov or crosshair you like, perhaps a certain r_wateralpha setting --- and Nehahra will smash your config to pieces.

(Zerstorer likes to do some of that as well. If you use Quake default settings, someone may not notice --- like fov 90, viewsize 100, r_wateralpha 1, crosshair 0 -- you may never know it happens).

Nehahra is difficult to deal with.

mh wrote a long commentary about Nehahra once. It was a good read.

I got obsessed a bit with Nehahra, pushed my stack of engine coding chip into the pot and said "All In" a few years ago and kept defanging it until I could play it and enjoy it with zero annoyances.

(Then I think I stopped engine coding for 2 weeks and played Nehahra with some frequency -- -haha. Nehahra really is quite a masterpiece and it becomes clear after the annoyances are eradicated). 

Recorded a playthrough on "+Quoth Nightmare", i.e. "skill 3; coop 2". A while ago actually, but youtube was being fussy. In 4k or as demos.

IMO that the coop flag makes this masterpiece even more masterpiecey. Gugs and droles are less exploitable and a fairer challenge (esp. in ambushes) than shamblers, extra deathlords make ammo management a bit tougher, and I prefer gaunts to vores as well. Vorelings don't work as tarbaby replacements at all though.
It isn't fully coop-compatible: GL secret in warpa softlocks warpb, and half of warpd can be skipped by picking up the same keycard multiple times.
Other than that, what's there to say? It's still the best.