This is more or less my take, but at least for me it's not about the level of difficulty exactly. It just feels very transparently like exploring a state space, where the "a-ha" moments just boil down to breaking into a different neighborhood of that space. To be clear, the puzzles are very well designed, and also very hard; but I don't find solving them satisfying at ALL. Compare with Baba Is You, where the "a-ha"s feel to me more like having a grand insight than finding the right very specific sequence of moves.

Put another way, it's been years since I played Baba and I can still remember the key insights to some of the sneakier puzzles. I couldn't even begin to do that for SSR.

That's my verdict on Stephen's Sausage Roll too. My personal favorite sokoban is Patrick's Parabox (the name is an obvious nod to SSR) because its puzzles have a gentler and finer-grained difficulty ramp.