I mean technically every FPS is a grind. You shoot an enemy at minute 1, and then you repeat the same thing a million times until the end credits. RPGs can be kind of the same, even some tactical turn based game can be a grind if each battle is mostly doing the same things. You could say all RPGs are one big grind if you don't like all the running around and reading shitty quest text and then doing your lame little combat routine on 1000s of goblins or whatever.
It is all about the context. One man's tedious ass grind is another man's Underrail.
The difference is that in your typical quality FPS game you meet different sets of monsters in different locations and as you progress through the game the difficulty is steadily increasing, throwing more complicated encounters at you. Dooms don't have leves where you walk into a flat room, shoot 3 imps, walk into another flat room, shoot 3 imps again and repeat that for an hour or so.
Everyone would call it out for being an unimaginative boring mess. Yet that's exactly how grinding and some RPG dungeons feel like.
Yeah the problem with grind is that it makes you repeat the same encounter over and over, or repeating similar randomly generated encounters over and over, with little to no variation in enemy constellations and environment.
In a good shooter, encounters are made up of
a) the monsters
b) the environment
Proper level design is important. You don't just vary the amount and type of enemies, you also vary their placement and you vary the environmental hazards.
Fighting 5 melee enemies on a bridge leading over lava is far different from fighting 5 melee enemies in a boxy room with no hazards in it. Similarly, engaging a bunch of flying enemies in an open an vertical space is a lot different from engaging them in a narrow corridor where they can't make use of their flying ability.
This is why FPS games only really became good with Doom, and most so-called Doom clones of the 90s (which were more like Wolf 3D clones) failed to be as fun as Doom: Doom introduced height levels and therefore verticality into the genre. Now you can have stuff like jumping puzzles, enemies attacking you from above, flying enemies like cacodemons, etc etc. Wolf3D and its clones had no choice but to give you ever-samey featureless square rooms due to tech limits.
RPGs need to embrace environments as a factor in encounters much more. Some games do it pretty well already: Battle Brothers has height advantage, some types of ground tiles like swamp that cost more AP to traverse, etc. Blackguards did a lot of interesting things with its encounters, giving each one some unique mechanics.
The more possible elements you can add in an encounter, the more interesting it will be. Variation is key. Oldschool dungeon cralwers spiced up the dungeon crawling by adding environmental puzzles, like having to use the jump spell in old tile-based Might and Magic games to bypass trigger tiles that would close a door.
Grind is pretty much
anti-variety. Grind is the same encounter in the same environment, over and over.
Dragon Age Origins had an encounter design that felt like a grind to me. Every dungeon had a dozen rooms that all contained the exact same encounter with no environmental variation in it, either: 3 melee + 2 ranged + 1 caster, copypasted over and over again. No creativity in its design, nothing fun, just rote repetition of the same generic encounter. That's what grind is.
