serious flaws
1. I accidentally picked the wrong gun to use, and once you leave the Shop screen and go to the mission briefing screen, you can never go back to change your gun. This was a problem because I had no chance of beating the next level with my weak gun, which meant no chance of advancing in the game. You need to give players a chance to go back to the Shop screen, either on the mission briefing screen or from the main menu.
2. What's the difference between the most expensive 2 units and the cheaper units, since the statistics show that they're not any better?