The difficulty of level 1 seemed normal. I guess it depends on what the player is expecting. Not much easier or harder than games like Metal Slug and Alien Hominid, although the boss gave me some trouble. On that topic, I found myself dying a few times to the snakeboy when he came up from under me, especially in the middle of the screen. You can get used to it, and learn when he's going to come up there, but it's almost impossible to react to the first couple times. The second boss does a great job of telegraphing the slam while still being hard to avoid. The checkpoints do a good job of letting you get back in the fight without making you feel like you cheated when you win.
Final thoughts: I'd actually really appreciate a speedrun mode or ranking system to push me to 1cc the game. The former shouldn't be too hard to implement. Also, I still have no clue what the fuel pickups are. I guess they're bombs? Fuel might be a bad word to illustrate that