I think it suffers a bit of lack of ambition. For a game that has been in development so long people expected it to offer something special.
Instead it looks like a 15 hours game where most of that time is due to backtracking or being stuck trying to figure out how to progress. If you removed all the artificial difficulty it would be a relatively small and short game content-wise. In the end it nails the atmosphere well, but it doesn't do anything new with the roguelike genre. It's fairly simple, with an handful of mechanics. It never becomes more than the sum of the parts, and those parts aren't deep or even complex.
(I think Rain World is the example of a much bigger and more interesting game, that also didn't have the success it deserved, whereas I think the lukewarm reactions that Below got are proportional to the overall quality. It's good but not great. And it doesn't stand apart.)
It's a kind of small, charming game with some difficulty spikes and frustrating sections. Even if it's inspired by Dark Souls it seems like it only borrowed the shallower surface of it.
But without high ambitions it's nice, interesting game. It's just that I think people expected something more. Something that you could invest into more rather than play across a weekend and then forget it existed.