I finished the game in about an hour.
Honestly, I don't think it's that DIFFICULT, most of the difficulty is due to bad controls. No offense to the dev, but there is no momentum mechanic, so the character doesn't keep moving in the same direction when you jump off a moving object, or let go of the movement key.
in addition, there are a few jumps that don't appear to have been playtested, the biggest is one where there's a couple sawblades right next to a checkpoint (luckily) where the gap is too high to clear with a single jump, but too low to clear with a double jump, so you have to kind of do a double and then awkwardly fall through the hole.
The spinning parts were also terrible in general due to no friction either.
In addition, there were multiple parts that could be cheesed, most of the falling platform sections were basically skippable, most of the sliding puzzles allowed you to just flick into a blank wall and reset or skip mechanics, and the final boss has a spot where she can't hit you at all.
I'd give it a 6. I'd say it's not that bad but as the 6th game AppStir's put out, it really should be better