I'm sorry but this is not a very good game. It's repetitive, boring and way to easy. It is quite addictive though which is why I at least wanted to see all upgrades, because I thought almost all of them were useless. The only one I deemed useful was the machine gun, as the shotgun is really dumb with that delayed fire and the Uzi is just too weak (even fully upgraded). I was almost never hit and despite only upgrading life once in the beginning, I never died (except when using the shotgun on like level 28 or something).
The levels don't really seem to scale in difficulty, at least until you recognize that the machine gun is the way to go. I found it especially noteworthy that in the third or fourth row I did not encounter jeeps or tanks until the very end, only footsoldiers and mines which felt like replaying level 3. Which brings me to replaying levels. The stars do nothing but looking shiny. When replaying a level with one star and achieving three, you get nothing but the loot, so it's basically a waste of time for ocd patients.
I wasn't really going to comment on the variety of enemies and guns, but I accidentally reloaded the page and saw that the game was made in Unity. It is incredibly easy to add variety in Unity that not doing it is like a sin.
I liked the music, and the style reminded me of metal slug, so two stars.