The melee combat cannot be too complex as the game is also - in fact, mainly - a shooter (so you should not expect to, say, finish a combo before getting killed by someone from afar).
The melee specialist needs to be able to kill relatively quickly when he reaches a enemy, but we also want the enemy to have a chance to be able to get away.
Of course, we also want the melee combat to be meaningful. That means that we want the best player to win the fight, not simply the first to attack or to defend. We are always working to strike a good balance between all these constraints.