Unfaithful

As always, people need to be reading their Machiavelli:

Mercenaries and auxiliaries are useless and dangerous; and if one holds his state based on these arms, he will stand neither firm nor safe; for they are disunited, ambitious, and without discipline, unfaithful, valiant before friends, cowardly before enemies; they have neither the fear of God nor fidelity to men, and destruction is deferred only so long as the attack is; for in peace one is robbed by them, and in war by the enemy. The fact is, they have no other attraction or reason for keeping the field than a trifle of stipend, which is not sufficient to make them willing to die for you. They are ready enough to be your soldiers whilst you do not make war, but if war comes they take themselves off or run from the foe...

The mercenary captains are either capable men or they are not; if they are, you cannot trust them, because they always aspire to their own greatness, either by oppressing you, who are their master, or others contrary to your intentions; but if the captain is not skilful, you are ruined in the usual way.