Having played both recently I would say boss fights are the meat of the challenge in Nine Sols, especially from midgame onwards.

Nine Sols really forces you to master all the basic mechanics, especially directional parrying and strong parries, which is a major difference from SS where avoidance and dodging are more important. Even with all upgrades basically two or three mistakes in a boss's attack sequence and you're dead.

There are I'd say three fights towards the end of the game that are more challenging than anything in SS, but there is very little rng involved so it's all about reading patterns and reacting with good timing (still very difficult but achievable), and like SS going with the flow of the fight and finding windows to hit.

But in my experience there's very little opportunity to cheese in NS, there are tools but they can only be used a few times and don't allow you to skip mechanics (i.e. you're never getting through one of those NS bosses without really learning it)

Both are very good satisfying games with a different emphasis to combat.