Jump is slow and low and not working properly, the dash is just not in the game and that's really hurting the gameplay. Boss deals just too much damage. Every mistake just sets you back on another minute of gameplay that really drags the game in the no fun zone really fast. Lack of music and any sounds whatsoever not helping to effectively fight the boss too.
Like the minimalistic visuals and UI of the game, but there are really big balance issues that you need to consider!