Preface: I have played through DS1 multiple times, DS 3 multiple times, Elden Ring multiple times, and Sekiro twice. I love all these games to bits. I tried DeS, and while I found it fascinating, I came upon it too late and did not feel like proceeding as I have been spoiled by later titles.
Sekiro is beloved due to its very tight design. It asks the player to excel at a handful of specific skills rather than presenting a wide array of options and going “well some of this ought to work for you if combined correctly”. Is either approach better than the other? Subjective. But it’s easy to see why one or the other could appeal more or less to individuals.
That being said, with a narrower band of skills to sharpen comes more constrained encounters. Sekiro is (chiefly) a game about one-on-one combat where nearly every attack can be deflected. “Parry” really is the wrong word for the primary defensive option in Sekiro. Even attacks that are telegraphed with the big red “watch out!” warning actually CAN be deflected (though perhaps you’d be a fool to try). The game is clearly trying to get the player into the groove of trading strikes. You attack until sparks fly, the enemy disengages, or winds up an attack unhindered by your strikes. Then you are met with the defensive challenge: here is one or more attacks with different timings and potential responses. It is now up to the player to answer with whatever they feel is most effective. This may be deflect, block, dodge, jump, mikiri counter, consumable item, or shinobi prosthetic tool. You may find more than one answer fits and therein lies the player’s ability to be creative.
I would argue being creative within a more limited set of constraints does not necessarily diminish the quality of satisfaction one could experience. Being able to master - or at minimum become proficient with - a difficult set of skills despite constraints could even be argued to be far more satisfying as there is less opportunity to find some cheese in the massive toolbox that you would otherwise have access to. And it’s not like the toolbox you have in Sekiro is as small as some detractors seem to think. However, it is much smaller and focused than other titles… especially Elden Ring.
And this is me only talking about the combat. All the other aspects of the game are very well designed too, but I’ve said enough. Also, critics tend to focus on the combat anyway.
I experienced few bugs in Sekiro, so I can’t comment on that. The buggiest/jankiest experience I’ve had with a From game was DS1 and that stuff did frustrate me to no end.
The difficulty spikes you speak of for Owl 2 and Demon of Hatred are kind of the point (you didn’t mention it, but Butterfly 2 as well). Those are optional story-focused bosses where everything is cranked up to 11. They are “challenge mode” encounters. This applies to Demon of Hatred especially. It’s basically a DS boss in Sekiro. The boss mechanics and combat mechanics clash with dissonance. It makes it challenging, but in a rather frustrating way because few other encounters are like it (maybe headless to a small degree?). You’re basically taking a test for a subject you didn’t get to study for so you have to learn on the job. It is definitely the fight that took the most attempts by a wide margin. Conversely, the final boss is the opposite: the culmination of all the things you’ve been taught put on dazzling display. It’s your final “performance” if you will.
None of this is to say the skill ask of the player is not demanding, because it is. The patterns required to respond during enemy aggression is different than a Soulslike, but I think it’s still in the same neighborhood. “When the enemy/boss does X, I do Y”, the difference being in Sekiro the strings of X and Y are much faster and there’s far more of them. If you don’t like that variation on the theme, well then that’s it… You don’t like it! There’s nothing wrong with not liking something. And I know some players describe an event while muddling through where something “clicks” and they suddenly “get it” and then enjoy it far more (this applies to all From games). But it’s possible Souls games will click for you and Sekiro simply will not.