Simple, Needs Tweaking
This is a fun game, but some mechanics issues got in the way of it being great.
The mechanic for the unicorn shooting powerup needs to be tweaked. I once ever got the bar beyond the second tier (large blast, small side blasts), for about 1 second. Most of the time I'd have it above the first tier (large main blast) for about 3-4 seconds before it dropped down again. If you're going to have the bar decrease so quickly, you need to increase the drop rate for the upgrade, or otherwise it's pretty pointless.
Personally I found the bad guy's multishot very difficult to dodge, and I think that sort of thing would be better suited a little farther into the game instead of the first shooting enemy you encounter. It doesn't really fit with the theme as well in my opinion. You'd think the unicorn could get the drop on humanity at least in the first level before they'd have mustered shotguns or whatever it's supposed to be.
The game's simplicity is fine, except that its simplicity means that it doesn't have anything to make up for the above problems.
Additionally, on the first level I was instructed on how to move and had enemies appear before being told how to shoot. Consequently I ran into my first two enemies (not knowing shooting was an option) and therefore started off the game with a quarter of my health gone. I would recommend putting your instructions in total at the beginning, or at least not have any enemies show until the instructions are displayed. A minor annoyance, but something to improve upon in the future.