The game stablishes a clear loop from the beginning, which is an invitation to wonder what will be the next tech you get. Also, stages are not too long so it doesn't get tedious.
Aesthetically, I feel the color palette elicits peacefulness, even if you are constantly killing. So the chill vibe of murder helps the player to stay more without feeling too violent.
In terms of balance, I think nature is noticeably more powerful than the protagonists, so after a while it gets overwhelming (even if you are using a rifle against unarmed creatures). It might help to make the weapons more powerful or add some mechanic that helps dodge creatures (dash/jump/shield).
The UFO leit motif is surprising the first time you hear it. Even without seen the UFO, the game is already telling "yep, ALIENS" which stablishes complicity with the viewer.
;)