This is a nice game. I liked the graphics and gameplay loop. The controls were nice and tight. I enjoyed it a lot.
This may sound nit pickey but try to stick the the theme completely. By that I mean use an appropriate font and screen effects. Consider adding some Chromatic abberation (in the unity post processing stack), Bloom, Film grain or some Scan lines ( even I couldn't figure out how to add these to my game :( ) to make it FEEL like you're in an arcade on a machine or maybe even on an old computer
The sound design was cute however it was very clear that it was just you going pew. Maybe up the bass and distort it a little. Try open source sounds or get creative. Slam a door shut for an epic explosion or slam down on a stapler for the nice metalic clang of lazer ricocheting off a metal hull.
Another issue i had was regarding 'game feel' Mark brown made a nice video explaining it (linked below). Add some screen shake and Bloom (it is in the post processing stack made by unity on the asset store). Make the bullets pop and feel important. Consider using a particle system to make them glow and pulse to look vicious and dangerous. When the ship gets hit, make it flash or look like it got affected. That's in my opinion what will push this from interesting and good to great.
P.S: I know this is long and mostly negative. However, I really love the game and i see great potential. That's why i care enough to write an essay on your game.