Shots cut off seemingly once every 3 times fired. They never hit targets when that happens.
After defeating 3rd boss, plane flies off to the left and the game never progresses.
Money is very easy to come by once the ship is upgraded further, which takes away some challenge.
I'd recommend being able to have 'slots' in the ship where you can drag and drop upgradeable items.
 "Frantic Sky"){kind=icon}
 "Galaxy Siege"){kind=icon}
 "Zombotron 2"){kind=icon}