Overall really fun, I might be a bit biased because these sorts of build your ship/vehicle games are one of my favorite types of games. Especially when there is physics interactions like this one. I liked the fact that good thruster placement was critical to your success, I feel like that makes for an interesting challenge.
Later in the game you don't really get a chance to stop and repair damage, so my ship kind of turned into a mess of turrets and thrusters haphazardly strapped together in the few seconds between fights. Fair play if you're going for a more chaotic style which definitely works well with the whole salvaging parts thing. But otherwise maybe adding some sort of delay for the next enemies after the player collects salvage or kills a good few enemies.
Balance feels good, in the later game you can basically shred through enemies but taking hits can easily mess up your weight balancing so there's still a good challenge to be had.
The background was beautiful, just I felt the parallax effect could've moved slower in relation to the player. The current rate gives a good sense of speed but feels kind of weird when the background is flying past and you're slowly moving towards/away from enemies or salvage. You can really see it if you move your mouse back and forth on the screen.