Too hard
The difficulty curve in this game is off the charts... I don't mind playing difficult games, but this game gets way too hard way too quickly. It wouldn't be as bad if the player was able to keep all earned money when he died, but the save file is reloaded, which means all money earned is lost. In a game like this, the player should not have to grind out the starting level just so he can progress to the second one. Also, money earned is very low considering the cost of upgrades; 600 bucks for one level, and then most of that is spent on repairs.
In later stages of the game, it is just simply impossible to survive; there are so many ships with shotguns, missiles raining from the sky, homing missiles, and torpedoes that most shots are blocked by the enemies' munitions. Those projectiles not blocked do a significant amount of damage, and since they arrive en masse, the player doesn't just take a few points of damage, but loses large chunks of health at a time.
I really don't think it is possible to beat this game; the levels get too hard too quickly, there are too many bullets/missiles/etc. on-screen at a time, and the player just takes too much damage. This isn't a bad concept for a game, but the implementation needs a lot of work.