Of course before I get into it I would say that I enjoyed Sekiro very much and consider it to be a great game. But it has its fair share of shortcomings (for me personally):
*Combat is restrictive and one note. The better you get at Sekiro, the more limited you feel because you eventually realize the optimum way to play.. VERY quickly into the game. The execution level is not that high either so it's all a matter of learning enemy patterns because Parry timing is almost mashable with how easy it is. Because execution level is low and combat is restrictive... that means that while the combat has a high skill floor, it has a low skill ceiling. Eventually anyone who is decent or good at Sekiro will all just play the same to each other. There is nothing to improve on in the game aside from parrying every enemy/boss attack consistently.
*The Consumables + Combat Art /Ninjutsu+ Prosthetic trifecta of tools sharing a SINGULAR resource pool of Spirit Emblem further restricts your options and in general is very poorly balanced. It would have been quite fun in combat if you had a separate meter for combat arts and for Prosthetic Arms but because they are shared, you just want to use the best thing out of them which usually just means using the best Prosthetic and a free but powerful Combat art like Double Ichimonji. The best set up in the game is Double Ichimonji plus Long Spark Firecracker, tools which you can get very early on in the game and can carry you through 90% of the game INCLUDING the very final boss. Many of the combat arts in the game are straight up trash and consume too many Emblems to ever be good. Many of the Prosthetic Tools are at least situationally good but the resource limits how much you can use. Rarely if ever you are going to be using 3 different Arms in a fight.
*Tons and tons of mini boss reuse. Many of them you fight 2-3 times, some even 4+ times like the Longswordsman, the Headless and the Shaman. The mechanic of using Divine Confetti of turning the spirit minibosses into a joke is just simply lame. They also reuse some bosses as well. The double Guardian Ape boss fight is tedious and lazy as all hell. And to be honest, there are only a handful of very good bosses in the game that are also unique. Lady Butterfly, samurai on a horse. first Guardian Ape, Genishiro, final boss... maybe Corrupted Monk if it weren't for stealth insta kills. Demon of Hatred I despite because its like a Dark Midir fight in Sekiro which is a poor fit for the game. Lots of hitting and running around against a tank of an enemy who can just random two shot you and has terrible hit boxes.
*There's quite a lot of backtracking/reuse of areas as well. There are 3 times where you go through Ashina Castle and getting the bonfires again. Two times where you go through Hirata Estate.
*Bonfire placement is far too generous and feels like you are always a couple of minutes away from the next one. There is very little interconnection within levels so you aren't really unlocking short cuts, you are just going through the level in a linear fashion with very little branching paths. I appreciate that the world is at least interconnected some what.
*Stealth mechanics are half baked as fuck. The enemy AI ranges from "I can't see this guy killing my partners right in front of me" to "I can see this guy hiding behind a building 10 stories above me". It's very pedestrian PS1 era stealth mechanics (yes you read this right, we had games like Tenchu and MGS1 in PS1 that did Stealth better than Sekiro) and its embarrassing to see in a Souls game. Luckily the combat feels good enough to carry the game because if this was a dedicated stealth game I would have dropped it around the Hirata Estate portion of the game. Stealth is generally just a mechanic for players to mitigate the difficulty of the levels and minibosses somewhat.
*Weird difficulty curve and spikes, the hardest part of the game is the first few hours when you are learning the mechanics and your stats are poor. Once you get past the first 3rd of the game and beat Genichiro... the game for me was a coast until I got to Owl and then the final two bosses (one of which was optional). Some of the miniboss encounters in this game are harder than some of the bosses.
*NG+ in Sekiro is in general a waste of time and is easier than your first walkthrough. You need to ring the demon bell and not have the Kuro charm to get a proper new difficulty experience and that means that you will be taking damage even if you just block. But it doesn't teach you anything you didn't know before, it just punishes you more. Like no shit you have to parry everything, enemies/bosses still die efficiently to the same strategies (ie. Double Ichimonji plus Firecracker spam). I learned this in the first 2 hours of the game... I don't need to be doing this for the next 20 hours. There is no original content in NG+ either, no changing of enemy positions or patterns. You can just go for a different ending to fight the different boss... and that's it. A far cry from the other Souls games IMO.
*Story is fairly weak, Sekiro's main protagonist is bland as they come and the dialogue is nothing home to write about. No one should experience this game on English VA, it's low tier anime dub status. The story and plot generally put me to sleep. There was a lack of intrigue, atmosphere or mystique in the story and lore, things were fairly straightforward but not presented or told in an interesting manner.
*The music and atmosphere of the game was surprisingly not memorable for me. Not that it was bad, but I don't remember any track in Sekiro except for the one that blares when you get spotted in the first few areas since you listened to it so much. And MAYBE the menu music.
*Camera turns to shit if an enemy jumps over you or if you are in a confined space or near a corner. The Longswordsman fight in Ashina Reservoir is the prime example of this.