The big issue is that they look too flat. All shapes have one-pixel-wide highlight at top and one-pixel shadow at the bottom, which makes them look flat, all parallel to the viewer, and 1-centimeter thick. You need to use thicker highlights and shadows to represent shapes that are thicker : ship bodies, reactors, turrets...
Darker shade can help represent something that is farther from the viewer, and lighter shade when they are closer.
Last, you probably shouldn't keep the internal black outlines. I know they help early on when designing the general shape, but as soon as you need to flesh out the volumes, they get in the way: they make a huge separation between forms that are actually contiguous.