Having a number of save slots would have been nice, but this game is actually very, very nicely made. And despite the complaints you seem to be getting in terms of difficulty, I think it's just fine. Getting no money until a level is complete forces you to really think about what you spend your money on, and unless if you've completely messed up in a really bad way, most early are still beatable if you just play well. I don't see any problems with enemies being able to fire at you from offscreen, or the fact that they get stronger weapons before you do; if neither of those things were implemented, the game would be so, so easy. I think that making the enemies that way was a very good decision, and judging from the menu BGM and the main menu background, this game wasn't meant to be easy anyway.
Starting from level 15, the game starts to make you feel a bit helpless with how strong the enemies become, but again, that is a good thing. This game, and bullet hell games in general, absolutely has to have moments where the player is shocked by how powerful the enemy is. And the thing is, despite the fact that enemies can fire salvos of 30+ missiles at a time from level 15 onwards, every level is still perfectly beatable by a decent player who knows how to approach these insanely powerful enemies.
By having the difficulty the way it is, you've managed to accomplish something that the majority of other developers have never been able to do. This game actually forces the player to exploit every little advantage they have over the AI enemies; most games like this end up being very easy because of the player's access to a radar and the fact that the player can move very quickly and very flexibly. However, with elite enemies that can drop your health from 100 to 0 in less than a second, being able to exploit those things does not make the game too easy, and that is a very good thing.
tl;dr: The difficulty is perfect. Don't let other people get to you, they just aren't good at the game. I look forward to your future games.