Look, I've already parroted what I'm about to say in similar discussions and in much greater detail so I'll try to keep this simple.
For Dark Souls the easiest route would be to compare it to the similar but better Blade of Darkness and Nioh.
The reason why DS is just decent it's because it has
essentially nothing to master. The biggest differences between the weapons are their stats but
their actions are so similar, so basic, so limited that aside from those stats there is basically no distinction between playing with a longsword or a katana. They fundamentally make no difference on how the player approaches the different challenges. That isn't something that happens on BoD or Nioh, where each weapon type is indeed very distinct in use and behavior, which makes the player think differentely the on way he acts. In BoD since even weapons of the same category have their own unique moves with different intentions, the game therefore has even differences between weapons of the same type. And in Nioh you can craft your own moveset, selecting out of a dozen moves for each weapon type, all with distinct properties, so that game takes a step further by demaning you to seriously ponder about the composition, synergy and purpose of your weapons choice.
DS is pretty much just a foundation for a combat system, a solid foundation but just a foundation none the less, barren of any other structure that builds up on it, to offer a truly engaging combat experience.
Action games are all about the tools given to you (how they play with each other, with the enemies, how they change your playstyle, etc...) and how to properly read and react against it's challenges/enemies.
Sekiro is bad, because it completely fails on both accounts.
The game is super mechanical.
By this I mean that enemies seem to be designed to counter all your attacks except those specifically designed to kill them - i.e., if an enemy can only be defeated by counter slashing, then that’s it. You can even see this in how weirdly limited/specific in use most tools and moves are. The design difference between Sekiro and Soulborne titles comes down to this - in previous games, a boss fight was dependent on both the enemy and your own build; here, the only thing that matters is the boss patterns itself and not the player.
The only way to win is to know ahead of time what attack the enemy performs and how to counter them. That's I memed alot about how the game feels like a rythm game, because the whole thing really is about performing a specific sequence of X maneuvers perfectly when dealing with Y situation.
There's no room for experiementation or improvisation. You either dance to the devs tune or get fucked.
Another proof of this overall poor design is how the game has quite a lot of "trick" moves just to fuck with you, where the same starting attack animation can lead into different combo patterns.
I'll be quoting
Lutte for this next example:
Take for example Owl Father's charged move. He does a charge that makes it look like he's going to dump the sword on your head, and that is what he's going to do if you don't move at all and dodge at the last second only. But if you try to just walk on the sides or dodge early, that move suddenly turns into a full horizontal sweep. If you dodge to the back he jumps on you. It's pure trial and error understanding that he can do the latter two moves, you know it because you made the mistake of trying to find alternative ways of dealing with that slow ass boring charge move. Then you learn that there's only one valid way of dealing with it: dodge at the specific timing.
This is not so different from the enemy uber tracking found in the latter souls games, that people complained a lot about, where the enemy would spin like crazy to follow where you are, it's just better animated so that instead of rotating in place the character looks like he's doing a different attack. And this is where you spot how much blind fanboyism this game receives, some of the features that were most complained about other From's games is now something taken as a positive.
The game is literally wants you to do nothing and only avoid it in the one, correct way. There's no opportunity for finding interesting positionings and having fun with the combat system. The game just wants you to wait and do nothing like a good boy.
Add to this the same weakness from DS, and seriously gameplay wise Sekiro is a shallow action game.
Again the only reason you guys think otherwise is because you haven't played anything better, plus the fact Sekiro has an amazing game feel.
You know what? Fuck it. I know I'll get shit for this but I'll say it anyway.
Sekiro is like a reverse Stalker. Amazing game feel and polish that immediately draws you in, but once you play it enough to understand how the game truly works it loses all it's appeal and you can't be bothered to boot it up ever again.