I think the difficulty is perfectly fine for what it is. And the game gives you so many "Bandaids" to patch up what one may lack in gameplay skill, with the shop. And 1ups are cheap!
I'm trying to go through the game without using E or M tanks, and doing it like that, it feels like... Mega Man 2. -shrug- A lot of the deaths my character receives feels like I'm lacking in patience, rather than the game just being blindingly hard. Especially with the bosses. Concrete man was a Joke. Splash Woman has some predictable elements (Like when she calls her fish, they come from the side she's closest too), and she gives you some good "free hit!" moments if you just wait for them, and Jewel Man was a lot easier once you focus on his weird quirks, like how he only jumps when YOU jump, and he doesn't so much follow you, as just... go from screen side to screen side.
Difficulty due to tight design, and not because of screen-filling unblockable projectiles, or overwhelming numbers... It's just hard to get this feeling from most modern 3D games.