I couldn't find a proper place to put this.

How well does the 40kRPG system hold up as a fantasy game? I know DH was basically a hack of WFRP 2e and my D&D group's usual slog through boring rules had me wondering about this. If you strip away the guns and fancy cybernetics and keep melee weapons, armor, and maybe psychic powers, then does it work well? I'd love the lethality of things.

Note: I'm not asking for someone to tell me to just play WFRP. For one, I know nothing about Fantasy's fluff and don't want to learn it either. This would just be a simple rules hack.

I think it's best to compare it to wfrp 2ed for now.

Close combat here can be extremely brutal with right talents, but it is so to balance firepower of guns. In wfrp there was much less options for melee but also you were quite limited in ranged combat. It'll be hard to balance two swords lightning attack with a bow (not saying that it cannot be done). Psychic powers are also improved in damage and casting ease compared to wfrp.
There are things that translate rather easily and fit nicely (like swapping damage dice with your degrees of success) and there are some that don't.

Could be a really interesting project, I loved the first edition WFRP, never read the second but the third edition looks really off putting to me, the part board-game element was the main reason for that. The bare bones combat could translate really well of course and plenty of the psychic powers like Pyromancy could translate over well into the various Schools of Magic in a Warhammer setting. I'd consider making primative ranged weaponry as lethal as lasguns etc are currently so they're on a par with melee, giving crossbows, longbows and handguns that deadly punch they deserve.

Maybe the most work would be in creating decent and balanced archetypes?

I love Warhammer fluff, the Gothic Germanic Renaissance take on fantasy is very cool and the Empire is one of my favourite's. With all this BC stuff expected to come out I don't suppose it'd be too hard to adapt a Chaos orientated system either. I know you were just talking in general terms but Warhammer is still more interesting than Faerun in my humble opinion.

DW

I've actually pondered that question extensively, seeing how my WFRP 2e campaign is barely holding together due to the system collapsing miserably under it's own weight at the higher end of the power scale.

Whatever ranged weapons you'd like to allow - slings, bows, crossbows, flintlocks perhaps - would need to be thoroughly revised to hold it's own compared to melee. Since we're discarding all hi-tech stuff (at least before magic comes into play), I'd say bows should start at about the level of a lasgun, and if early firearms exist, they'd need to be at about the level of a human-sized boltgun to make up for crap rate of fire.

Primitive quality is obviously a moot point in such a setting.

I'd strongly advise against using BC psychic powers as a baseline, they are clearly balanced against the idea that a non-psyker can grab a hi-tech heavy weapon.

All in all, it'd take some work, but might work out in the end.