Answer:
Pros: Mercenaries can often help alleviate the urgent need for trained manpower in a prolonged high-casualty conflict which can also have the bonus of not creating too much dissatisfaction amongst the populace against the leader. History even gives us an example of effective mercenary armies.
Cons: Mercenaries have no loyalty whatsoever even to their employer. Mercenaries are never particularly the kind of soldiers that a leader should ever truly employ in an actual battle, mainly because mercenaries throughout history have had a habit of deserting their employers when an actual