Like seriously, I was finishing 5th level on Hard diff when I realised I could've been playing it on Nightmare, as my health is capped at 50 anyway, and the power-ups appear to be increasing this cap further on Nightmare. I completely forgot the remaster changed the way Nightmare works. Is there anything I'm missing, or does it really look like devs' omission?

Sure but in Quake the difficulty setting also affect the amount of monsters.

For example, first level has 72 on Easy and 107 on Hard.

Also, one of the developers said that Nightmare difficulty in Dawn of the Machine alters some other stuff like faster Vore projectiles, less pain chance in monsters, and also you can't forever bait Shambler into melee attacks doing "the dance", etc.

Plus apparently there's also Bloody Nightmare difficulty that is hidden in a secret area halfway through an episode which also resets your inventory upon entering any level.

The number of monsters per level is the same between Nightmare and Hard from what I could tell so far. However, I was not aware of the Bloody Nightmare thing + changes in monster behaviour. That's actually good to know.

Your health gets upgraded on the hard difficulty same with ammo unless you cheat

Why tf would they put a health cap on hard?

HP cap is there even on Easy/Normal which is even weirder

I see from another comment that health gets upgraded. Thats lame, should only be for nightmare.

I agree. I don't see a point why Easy/Normal players shouldn't be able to upgrade their HP beyond initial 100 from the get go. This feels like a really poor experience for casual players

When you pick up the upgrade it says you feel revitalized. It's implied ranger has been fighting for so long he's tired and worn out.

Pickups are a lot more forgiving and in the first few seconds of the first map you get a upgrade to 60 health, so it isn’t too bad. And most likely this isn’t your first time playing quake given that it’s the 4th expansion.

There are still people who would want to run through it with as little hassle as possible, for whatever reason. It shouldn't matter whether it's 4th or 100th expansion - things should remain somewhat consistent.