There aren't any game-breaking mechanics right now. In terms of PvP, there is balance. Sure people get some freebies, but every class has their own thing that gives them an edge and promotes certain tactics. It's a big rock, paper, scissors game out there, with different weapon classes out-performing others at certain ranges and certain classes having a one-up over others in some aspects. The only reason why certain things are being used more is because they are more versatile in accordance with the kinds of maps out there currently and the type of game modes players choose to partake in.