Very cool concept! My feedback:
Art
I like the unified and simple art style, nicely done making it cohesive.
Bugs
When I died at the end, a menu appeared asking to retry or give up. I clicked restart and it crashed or something; the game became unresponsive and I couldn't continue.
Controls
It would be nice if shooting would continue on a timer when you hold the mouse button rather than clicking for each shot.
Game Design
I really like the design of the cargo units but I felt the "spread" shot wasn't worth it at all... until suddenly I was overrun and could have definitely used it lol.