Using your previous comment as a starting point:
You can do horror in most games as long as the rules don’t fight you.
I think you can do it even when the rules do fight you, as Ravenloft even existing through every edition of D&D shows!
The much harder issues imho are:
A) Player buy-in and willingness or even just ability to keep the tone/game on track enough that it can be horrifying.
B) Creating scenarios/adventures which are actually horror, and written well enough that they don't veer into just becoming some other genre, or just plain silly.
C) A DM who can actually evoke the atmosphere well enough for it to work. This isn't necessarily a high bar - I've seen new DMs pull it out first time. But I've seen experienced DMs screw it up, simply because they're too "matter of fact", because they're too focused on detail, or for other related reasons.
And these are interlinked, as well. Like, if we look at Mothership, for example, the rules aren't great, and I'm not actually sure they really do support a horror tone all that well. But what Mothership has is a huge number of genuinely good space horror scenarios/adventures, which are convincing enough that even fairly silly players often get into the tone quite quickly, if the DM can convey the tone convincingly (again, which doesn't require unique expertise or huge experience, but it does require some nuance and thoughtfulness).
I'd say most of the one-shots my main group does have been horror of various kinds, and some short campaigns too, and I've seen very wildly different success levels. Like, I've seen players strongly bought in, but a boring adventure which billed itself as horror but was both not very engaging, and a kind of trite, old-fashioned horror that just wasn't scary (and I kind of doubt was even scary when it was written in the 1980s). Equally I've seen players who weren't bought in at all, just joking around, gradually brought into a state of pretty real horror by the sheer persuasiveness of the adventure and DM (with Mothership's Bloom for example).
I think one important think for the DMs is, you've got to be ready to keep rolling when some horror element fails. Not everything will land every time, and you can't get caught up trying to make an element land when it hasn't. This is true in horror movies and novels too - sometimes Thomas Ligotti just writes a scene that's so gothic it's more funny than disturbing, sometimes the FX in a movie are unconvincing or the editing ruins a potentially a good bit of horror, and so on. But you've got more, so keep going. I saw a Mothership adventure a few months ago nearly derail because the "climactic" horror (or what the DM thought was that) just absolutely didn't land. The players shot it immediately from max range (like 30ft in that case), not even considering engaging with it, and didn't find it very compelling. But the DM did finally get over it and got us back on track and we had a good horror ending and so on.
One other thing actually connected to the same incident is "Don't be a show-off, and don't think you're smarter than the players". The adventure we were running, the person who had written it (not any of us, it was pre-written adventure) had decided they were so, so clever because they'd named the an NPC "Ghost" in Gaelic as their surname (and their full name came up a number of times), and presumably assumed - completely and obviously incorrectly - that the sort of huge detail-oriented nerds who play RPGs would never figure such a thing out, because that would give the game away massively, and make a "maybe" into a "definitely". Obviously we immediately did realize this (specifically, I blurted out "Wait doesn't that mean "evil spirit" in Gaelic or something?" - technically out of character but my PC's surname was Murphy so... and once the cat is out of the bag...). And this is a really common issue - over and over I've seen horror adventures undermined (often only a little but still) by writers dropping unnecessary flourishes like this. Like keep those hints and your ego in check, mate. Drop hints only when you mean to drop hints and it furthers the tone - don't try and show off, don't try and be like "OMG I GOT THIS PAST U" or "THEY WILL NEVER KNOW THIS ONE!". They will know that one. You're talking about people who are often obsessed with trivia and detail and words and etymology and often took odd languages at school/uni!
(This particular adventure also has the issue now that a movie is coming out with that name - the Gaelic word for ghost - as the title, so it's about to become even more obvious.)
Also, if you're writing and selling an adventure and you do put stuff like this - at least call it out to the DM so he change the name if he thinks the players will spot it! The DM didn't know about this little landmine, so he was vexed when we found it.
Don’t take away their ability to do things, make it so their ability to do things is the reason they aren’t already dead.
Yup. This is one thing some Mothership adventures do pretty well - they allow for the fact that the PCs likely have some pretty big guns, probably at least 1-2 combat-capable people in a party of 4, and that a lot of PCs will actually be quite skilled and good at their jobs - but that's the baseline, and the adventure is written on that assumption, so that just keeps your heads above water (in some cases literally!).