According to Machiavelli, mercenaries are disunited and without discipline; bold among friends, yet cowardly among enemies; they have no fear of God, and keep no faith with men. In a word, they are unreliable. If a nation, against his advice, relies on mercenaries for its protection, it keeps its ruin at bay only so long as it can postpone an attack.
The basic problem of mercenaries is that they are loyal to the wage and not the prince, and a wage is insufficient motivation for a man to sacrifice his life. This problem is even more stark when one contemplates the mercenary captain. If he is a capable man, he may aspire to prince-hood and take the prince's own nation by force. If he is not capable, then he will bring about that nation's ruin through his own incompetence.