Game Breaker: AFMBE is notorious in some circles for the sheer amount of damage that firearms do, particularly for a game where most of the players will be running baseline human characters. If you try running the game straight out of the box, a shootout with living antagonists is utterly lethal.
Nausea Fuel: Again, this is to be expected. Especially when you look at some of the zombie aspects, like Venous Defense (the zombie's veins and arteries poke through the skin as flailing tentacles), Entrail Whip, and Melding (lets the zombie fuse its flesh with that of a living victim).
Squick: Let's face it, in a game devoted to facing off against man-eating walking corpses, this is going to happen. The existence of the PHADE virus is pretty nasty in its own right.
That One Disadvantage: The highest level of "Coward" will basically mean your character will piss their pants and either faint or run away screaming at the slightest level of danger. In a game where you will usually be surrounded by hordes of flesh-eating corpses.