I agree with the OP.
What I look for in a Castlevania game is a high level of challenge, tightly designed levels and tough but fair boss encounters where quick reactions and recognizing attack patterns are required to be victorious.
SOTN has neither of those things. The map is large and open, but contains far too many superfluous corridors and rooms which are just mindlessly sprinkled with enemies. The constant backtracking doesn't add much challenge. It's for the most part a nuisance and results in a game that feels artificially padded. Also, the constant fiddling with items and menus messes with the pacing of an already bloated-feeling game.
Levelling up all the time and eventually becoming omnipotent means any semblance of challenge is obliterated. Observing enemy behaviour in boss encounters used to be essential to overcoming these foes, but it's hardly necessary in SOTN. Just keep levelling up and/or stock up on items and nearly all bosses can be beaten to a pulp without the player having to actually master a competent strategy.
While I eventually played SOTN to completion, I was getting bored of it even far before the halfway point because of the reasons I just mentioned.
I hope I haven't offended anyone with this post. This is just the way I feel about this game. I'm sure the things I disliked about it are exactly the things which caused other people to fall in love with this game. And that's perfectly fine.