Have similar issues I have with Death Trigger 2 for Unkilled, with the difference being that the game is far more shorter in sequence of playtime for every missions
The zombies just kept getting stronger with your guy start to spend more time waiting for upgrade to able fend the zombies rather than the time used to spend to play the game, with the challenge for bonus point demand the impossible for you to achieve
The MP relatively poor with duels that doesn't feel as engaging as it was expected to be, with the game isn't about skill but number of who have the more powerful gun more or less win the match before it even begin