While Sekiro: Shadows Die Twice swapped out the slow, methodical mechanics and abstract, obtuse story of Dark Souls in favour of faster, more active combat and a narrative that is (relatively) straightforward, it's still very much a FromSoftware game. Consequently, you can bet you'll be getting your a** handed to you by the countless bosses littered throughout Ashina.
However, Sekiro does tackle bosses slightly different from its Soulsbourn bretheren. Mini-bosses are far more prevalent, while there's only a handful of what would count as "actual" bosses in From's other releases. That doesn't mean it's any easier, though - these sub-battles are in many cases even more difficult than the big bads you'll be facing off against. Just ask anyone who couldn't get past the Chained Ogre right at the very beginning.
Even better, there's a good chunk that you have to seek out for yourself. You'll bump into most while out exploring, but it's always advisable to find as many of these guys as possible, as not only do they provide some of the best fights in the game, but they'll award you with essential prayer beads and other items that will be essential for the endgame.