From my own experience and reading some message boards about the game, the devs just made shit very unbalanced for the monsters. The Hunters need to be very coordinated just to beat someone who is moderately skilled as a monster.
A skilled monster player can pretty much wreck any team of hunters. If the medic is killed, the Hunters are fucked and will lose. Monsters move faster than Hunters, so Hunters are constantly getting hammered. Without the medic constantly healing the team, the team will lose. A skilled monster will dance around a bit and avoid major damage, get at least to stage 2 power, and then locate the medic. The monster then just needs to ignore the rest of the team and hammer the medic to death.
The rest of the Hunters can shoot away at the monster while it kills the medic. The best they will do is destroy its armor bar. The monster can then quickly dispatch the remaining three Hunters who have much smaller health bars and no one left to heal them.
The behemoth monster is especially bad. It's armor is nutso at stage 2 and 3. If a retard was playing the monster, you could literally spend 10 minutes just chasing and shooting it before finally winning. Meanwhile a good behemoth player maybe needs 3 or 4 minutes to crush the Hunters.