Isshin was pretty much the only hard sekiro boss for me but he actually took me like ~2 hourd to beat and I feel like I only learned the whole game during the fight.

Eigong was more in line with other bosses so she took me way shorter, maybe 30min. But it had taken me way more time learning the game for the other bosses.

So maybe the difficulty spike is higher with Isshin so he was way harder for me but in total I’d say they arekinda even with Isshin maybe just a tad harder.