You got me thinking: it seems like shoot-em-ups, more than any other genre in my experience, have this correlation between difficulty and quality. Whenever people really like a shmup it's almost guaranteed to be brutal and if a shmup is easy I always see people talking about low difficulty as a main negative that drags down any positives that the game might have.
I wonder if there's something about shmups that makes them rewarding only if they are hard or if there's just no casual audience for them and everyone only wants to be hardcore.