More frustrating then it has to be.
I'm not going to lie, difficulty had an effect on this score but its not for the reasons people might think.
The game when you start off is incredibly fun, fast paced, eye pleasing colors, incredible music. Everything a game should have in my opinion. But at later levels you really start to see the huge flaws that effect the game so much it becomes unplayable, and for me it was heart breaking to have to give such a low score to a Mario game.
To begin with, the hit detection/hit boxes need a hard refining and roll over. There were times when i more often then not just randomly exploded without hitting anything. This became so common during relaunch, and got so bad to the point where bullet bill exploded before he even let the canon and then a chain of events would occur where i would explode again and again and again before even leaving the canon.
Reaction time for moving Bullet Bill is pretty sloppy as well. In fact he moves just so slow up and down sometimes and I could not figure out why, In a game where reaction is key the game mechanics have to allow for those reactions to proceed otherwise its just unplayable.
Bosses in a sense are unfair, there is a difference between unfair and difficult, as mentioned before its a fine line, I deem it unfair for several reasons, one of them being the keys and the phantoms, yes you unlock something neat and amazing by doing it, but the phantoms move at such a high a speed, track and have an awkward hit box where they dont even need to touch you, instead them going near you is enough to hit you, that added with the fact that some boss attacks stay on screen and bounce of the wall, AND touching the boss itself when he/she is in her shell AND touching the ship they stand on itself ALL net you a death and restart, there's just no hope for some boss battles. This adds to frustration and overall subtracts from the enjoyably factor of the game.
Over all, I was sadly disappointed, And unfortunately this is something I am seeing alot in the newer NG games coming out lately, Developers focusing to much on the difficulty factor, they miss these small game breaking frustration factors which will eventually all add up and destroy their hard own work.