Ok but
I have a few complaints principally about the 3rd combat unit, Spiked Crawler.
1 - At some point in the game you need to destroy bigger buildings, and this unit in specific have an aiming problem. When the building is in its "final destroyed form" the crawler just can't hit it, at least not after missing lots of spikes, only one hits it and it's totally random. It could also aim a little higher because of enemies that fly too high.
2 - When the crawler is being chased by the bigger helicopter, if it goes all the way to the right side of the screen it will get stucked there waiting for the enemy to release the bombs until it dies. Like they move practically at the same speed, maybe you could think about reducing the bigger helicopter speed just a little bit or make the unit return after reaching the end of the screen.
3 - The big alien thing that spawns the units is pretty much useless, the blast wave could do some harm at least in the tanks and choppers. Or we could have some upgrades for it too.
Other than that I enjoyed the game, it's a nice concept and not really easy. Of course it is all about strategy and after you find what works for you, all levels can be beaten the same way.