This article explains why the emergence of mercenaries as a common thing in today’s conflicts endangers international relations and increases the risk of war.
The biggest problem of course is that one day mercenaries discover two things. First they discover their employers have no weapons, are not willing to fight and do have money. Second they discover that they have weapons, are willing to fight and do not have money.
The actions those discoveries cause are pretty much inevitable after that.
One might use the Thirty Years War as a fine example of the problems mercenaries cause and why they became pretty much illegal for centuries.