At the time, it was terribly over-balanced to favor the players in that it would often take several monsters working in concert to kill a PC. This was mitigated by the number of people who played monsters so we could outnumber them 2-1 or 3-1 and have some great battles.
Then people started hitting 50 (and a fully ranked lvl 5 monster was no where near a lvl 50 in power) and those lvl 50’s got bored with the PvE game after getting all their traits and the best weapons/armor and started flooding the PvMP game as Freeps. The PC armies would roll over the monster armies and it just became a waste of time to play. Which meant that people would log in a monsters, say “Oh, we’re trapped in the fort again” and log out. Went completely against Turbine’s notion that the monsters would be able to win victories by fighting as a swarm.
What Turbine needed to do (in my opinion) was directly rebalance the monsters & PCs against one another but they refused to even consider it. I don’t know if that ever changed.