I love me some Souls, but bounced hard off Sekiro. It has potential, but there's too much that doesn't quite work. The world has too little worth discovering, the game design is ill-considered and the combat feels too one note.
I don't think the bosses are outstandingly hard, per se, I just think that they're poorly integrated into the broader game design. You can clear most areas and mobs with relative ease, mixing stealth, deathblows and ordinary combat - most enemies can be guard broken in a handful of hits, so even spamming can save your ass. I didn't die much in the world, overall, because it's quite easy to mitigate challenge with smart play.
Bosses on the other hand feel like bullshit because they straight up fleece you of half your options on the way through the door: Verticality? No. Stealth? Not a chance. Grappling Hook? Only if you ask nicely. Then they push all the other stuff up to eleven and if you're some poor sucker who's been sneaking your way through the world, then by the time you reach Genichiro you're going to be in for a brutal time.
There are some off the track challenges, but you'll learn early on that most rewards are minor and so it removes the incentive. Dark Souls risk-reward design made optional challenges alluring, because they were normally worth it and this organically teaches you the game, but Sekiro falls down hard here. Think about the Seven Ashina Spears guy. Unbelievably tough challenge and your reward is a Prayer Bead; you bin most of your Sen and gold just getting through him and your reward is one quarter of an upgrade.
The limited used shinobi prosthetic and special techniques make things worse - if a game as hard as Sekiro gives me a weapon and says 'you won't always be able to rely on this', I'll pretty much never want to rely on it and that's what happened. Dark Souls says 'hey, Estus is infinite and there's no great penalty for death so just keep going until you get it', Sekiro says 'hey, you're out of spirit emblems, you're pretty much broke, all your vendors are coughing up blood and every time you fail, this gets worse'.
That so many people bounced off the game for such similar reasons, suggests this is more than merely academic. I honestly don't think you need to step back too far to realise how badly designed Sekiro really is - especially when you consider that all the things it manages to get right is stuff the studio has been refining in much better games for nearly a decade. One thing is fairly sure, that 10-years from now, you won't have an entirely new genre and paradigm in gaming inspired by Sekiro.