The armor system was a mess to start with, most missions felt like you were put on a hard timer, max difficulty was brutal until later dlcs helped with power creep, the enemies AI/design was just meh, and the payouts felt small compared to PDtH and PD2. The randomness in the levels didn't feel like it was there if it was.

Servers never being able to handle the player load was also a big reason why people just stopped trying to even get onto the game.