Lead dev of the game here! The game's main inspiration was Alien Soldier, which was another Treasure-made run-and-gun for the Sega Genesis... with some similarities to Gunstar Heroes. And hey, in fairness, I love Gunstar Heroes too and the game is a bit inspired by it too, heh...
Glad to hear you enjoyed the game! A steep learning curve is definitely something I'm aware is a bit of a problem in this one. If/when I make a follow-up I'm kinda hoping I can find a way to make it a bit easier to get into, though admittedly with the ideas I have in mind that may be easier said than done.