The card system isn't very complicated, once you get into it, it makes the game so much fun to play.
It allows the player to create VERY unique playstyles and it's the bread and butter of the game's replayability.
U can be a meatgrinder who chops everything down with an axe and heal off of ur kills. U can give up sprinting and ADSing for buffs and turn the game into l4d-like in the process. U can get infinite ammo on ur secondary shotgun and just keeps pumping out high dmg shotgun rounds like a mad man. U can be super sonic and go very very fast. U can focus on earning money...be a super medic...sniper....
That said, don't expect to get access to all of that at the beginning, u need to unlock the cards and theres a TON of stuff u will need to unlock...
No game is perfect and this game has its fair share of problems, but for the most part, it is very enjoyable.