Over the piece by itself works, but there is a lot to be worked on, even with your current style that could make it more impressive and appealing.
To start, the background could use a little bit of shaping up, I tend to suggest avoiding using the gradient tool to do large areas like this due to banding, which is where the computer has trouble displaying a clear transition because it runs out of colors to display. I would suggest a large airbrush tool, however, I would also suggest experimenting on your own to find out which kind of background works best for you. You do have a cartoony line art style with cell shading, another reason the gradient doesn't exactly belong, so I might suggest experimenting with that. Don't be afraid to redo things when they look wrong. I've had to completely redraw the background for one of my drawings because it just didn't look right.
Slowly coming away from the background, the UFOs are inconsistent with each other and could use some finer clean up to make them look more round. The shading is also backwards with then with the reflections needing to come from the lower right in accordance with the shadows on Duncan and Jones in the foreground. The beam of light coming from the one on the right could follow the direction of the ship as well, either making it straight down, or have an angle (since you're looking at it from underneath I would suggest this). The person also looks rather odd as well in this. It's no big worry, just a nit pick, drawing people from far away is a pain. As I can say both of us would be learning this, experiment and have fun drawing crowds.
The trails of the UFOs could be overall curved as well to create more of a dynamic look to it, most things do move in arcs overall.
On to the foreground. You do show some basic understanding of anatomy and have a good hold of proportions (Something even I struggle with) and match the style of the original content. So, not as much issue there, but I may experiemtn with cleaner and more consistent shading. Keep in mind your light source, which seems to be a bright light coming from the left of the image which would cast much longer shadows than currently visible. The shadows on the alien don't match the long(ish) shadows of the protagonists and it would actually be quite helpful to have the shadows to make the scene more dramatic. Overall, lowering the camera in the scene to make Duncan and Jones look taller than they already are would be helpful in creating a more dramatic looking scene.
I would be careful with the horizon line as well. It drops off right at the end, which is sort of awkward to look at. Feel free to bend the lines from the bullets as well. Increasing the DRAMA.
In summary, consistency with objects, shading, and working to improve consistency with your style. It’s not a bad piece at all and I enjoy how the characters are framed! It just needs some work in different directions. Excited to see what you do next! Feel free to contact me if you have any questions or need elaboration on anything I said