Thanks for the feedback! I agree that the explosions are a bit loud for when humans explode. They were originally supposed to be staggered randomly over a second or so; when I had to remove that feature due to C++ pointer jank it caused all the explosion sounds to play at once and stack the volume... whoops! Definitely something I will fix post-jam!
The penalty, which is about 1/3 of the humans, was indeed chosen to prevent spamming. In my testing, spamming the UFO with this penalty always leaves you with one or zero humans left, which was my aim so as to clearly communicate that the strategy wouldn't work. However I do agree it can be quite harsh when you have a lot of humans on the planet, so maybe making it a constant number (around 10 humans) could work better to make it less frustrating to shoot for a high score.
Anyways, super happy you enjoyed the concept and the art! Also, congrats on beating my high score :)