Wow, thanks a lot for the kind words! This is my first game, so it's really amazing seeing both good criticism and people enjoying it. I had a great pixel artist help out creating the primary assets (I did some of the art as well). Some of the resolution mismatch comes from that lack of experience where I misunderstood how to properly handle the sizing of the assets in Unity (although I figured it out towards the end).
I'm a composer / sound designer as well, and created this primarily to give me a game to mess around with audio. I had to crunch all the audio in 4-5 days, so I didn't get to write everything I wanted. Just 3 tracks ended up in the game - 2 ambiances, 1 boss fight track. My original plan was to have each boss use the same track, but change the instrumentation and poly rhythms based on their name (X-7, X-11, etc.). I'll give the audio an overall eventually.
I created a little trailer with the boss track if you wanted to hear it again: https://www.youtube.com/watch?v=TgHSQrD-SWw
Once I have time, I'm thinking of creating a sequel (COEUS 2) which would branch into a more Metroid Prime style.
Thanks for playing!