Dark Fantasy vs Grimdark

In reality, the "vs" referenced in the post title is merely a semantic one. I stumbled across this YouTube video, and the points that he makes are among many that I've made under a different label; how to do horror in RPGs, how to make dark fantasy, how to make "sword & sanity", etc. He calls it how to "grimdark" up your D&D game. I present it here for your edification, if you don't already watch this channel. It's pretty good stuff. He's probably a much better presenter than me (his hair is better than mine, and the tie and vest that he normally wears; good presentation, bro! I also have a tendency to struggle to describe what I'm talking about without wandering into tangents, getting overly rambly and repeating myself. I think his video is probably better than anything I could do on the same topic, even if I talk about most of the same points.

I'll make a list of the points, and my own commentary on them.

1. Use human opponents whenever possible. I'd add wild animals to the list. Keep our monsters closer to the vest, and use them more sparingly. My more detailed exegesis on this topic is here, from more than ten years ago now: https://darkheritage.blogspot.com/2010/11/making-monsters-scary.html Neither that nor this video really talk about how humanocentrism in your settings grounds it much better for "grimdark" or "dark fantasy" by subtlely influencing how your players see the world than one that's crawling with monsters and even widespread demihumans, for that matter, but it does. I think it's probably a question of heightened contrast.
2. Never use monsters straight out of the monster manual. I have a weird relationship with the monster manuals; I do prefer to, as he also says, reskin a monster or simply make it more difficult to identify if you have the kind of players who know all of the monsters and see identifying them to tactical challenges. But I also have always loved the monster manuals, and I'd never go so far as to say never use monsters straight out of it. Luckily, because I love monster manuals, I also have a lot of alternative monster manuals. The two Monsternomicon's from the 3e era are probably my favorites. This gives me a deeper well to dip into, of course. Anyway, my post linked above also addresses this idea.
3. Humanize the adversaries... including the monsters. This is one that I'm a bit more ambivalent on. I can see it's advantages, but I don't always do it, or see it as necessary except in some cases.
4. Make your NPCs morally gray or give them a flaw. This is part of the skulduggery vibe that I always tend to revert to, but this can be overdone quite easily if you're not careful! Players need to have NPCs that they can trust. At least some of the NPCs. Maybe they don't trust that the NPC has their best interest at heart, but there need to be some NPCs that they can at least trust what his predicted behavior will be most of the time. And if everyone is gray, you end up with a sludge of a world; grayscale is fine, but without some contrast, the dark in grimdark isn't going to be very impactful, so you need to have some pretty light gray, if not actual white, here and there too. Many times the flaw doesn't have to be a moral one, of course, as well. That isn't to imply that I disagree with this advice, of course. I agree with it completely. I just caution against binary thinking and doing it all of the time with every NPC.
5. The PCs themselves have to be vulnerable. True, very true. Plot armor ruins any game, in my opinion, but it especially ruins dark fantasy or grimdark games. My homebrew system has also eliminated much of the healing magic, as he recommends (although its replaced it with what I flippantly call "action movie healing" to faciliate play without long delays or gaps.) I can't imagine playing without taking that step. Even if you prefer PC continuity to disposible PCs, and there are a lot of very good reasons for prefering that, the players have to always feel the sense of risk. If they sense that there isn't any risk, any grimdark, horror or dark fantasy vibe is instantly ruined.
6. Start the PCs with nothing. This is an interesting idea. I've never actually done this. I probably wouldn't, except as a one-off idea just to try it for ambience once or twice. But I like the idea. And the idea of keeping hit point escalation from proliferating, and maybe even allowing PCs to start without more than a couple of hit points... that's certainly an interesting perspective, and I'm obviously familiar with it, but I doubt I'd be interested in that either.
7. Magic is rare and dangerous. Yep, yep, yep. I redid the entire magic system for my "alt.D&D" system to be overtly Lovecraftian in its mechanics (including sanity loss) rather than "Vancian" precisely for this reason. (Don't get me started on whether or not D&D magic is ever or ever has been actually Vancian. Maybe another time.) Magic-users should never be modeled, even loosely, on Gandalf or Merlin. The Ten Who Were Taken is a much better source for how to see them.
8. Travel is dangerous and should never be taken for granted. By the same token, I think giving thought to making overland trips harrowing (and interesting) is much better than "random encounters." Even if you use the random encounter tables at some point, you should probably do it before the session starts and give some thought to how to use what you rolled up and make it fit into what's going on the game so it doesn't just feel like random "it's time for some combat." Not that there isn't a place for that, and I'm fond of quoting Raymond Chandler's advice of when in doubt, have a man come through the door with a gun in his hand. But I do prefer more planned and useful rather than simply random encounters, because in reality the latter is kind of stalling because you don't know what else to do at that moment. Any encounter is an opportunity to reinforce the tone and mood, and should be taken advantage of accordingly. In addition to that, make travel dangerous for other reasons besides just bandits and monsters and wild animals. You ever been out overland in a terrible storm? I have. It's scary, especially if you're up in the mountains with sleet, lighting strikes hitting peaks all around you, and strong winds nearly blowing you off of your feet. You ever tried to figure out how to cross a rushing river that's running high because of recent rains without a bridge or reliable ford? Anyway, yeah—this should be a given. Travel should be a big deal, unless for reasons of pacing you want to do the Indiana Jones red-line across the overland map and fast forward to the next destination. Which, again, should be done to service the tone, the atmosphere and the mood; especially if you've got a tense, break-neck pace going on.
9. No fairytale endings. I dunno. Is Lord of the Rings a fairytale ending? Maybe, but everything was also bittersweet for everyone, even Frodo and Aragorn (and Arwen) and others. (And nobody calls Lord of the Rings a grimdark book; although in many ways it is moreso than people give it credit for.) This is a good idea, but it can also be overdone too. You want to give the impression that the world is dark and somewhat grim, but you don't want to give the impression that you're just screwing over your players all the time for no reason other than to mess with them. It's a fine line, but you have to know your players to land on the right side of it. Which, leads to the last point...
10. Don't overdo it. This is pretty key, and he says everything on it that needs to be said. Do enough of this kind of tone and attitude stuff, but don't do too much. How much is enough? Depends on the group. This is exactly why one of the most crucial GM skills, that rarely gets mentioned, but is, in my opinion, the most important one, is knowing how to read the table and react quickly accordingly. How do you learn that skill? Some people do it naturally more easily than others, but mostly it comes with practice and keeping an eye open for social cues.