Why does Machiavelli say that auxiliary forces are more dangerous than mercenaries?

Auxiliary forces are even more dangerous than mercenaries. “You get your ruin ready-made,” Machiavelli writes. While mercenaries are undisciplined, disunited, and disloyal, auxiliary troops “come to you as a compact body, all trained to obey somebody else.” Mixed armies are, of course, composed of both auxiliary troops and mercenaries. The ideal is for a prince to use his own troops. Cesare Borgia started out relying on auxiliaries (the French, specifically), and then switched to mercenaries (the Orsini and Vitelli) before resorting to troops of his own. Steadily, his reputation increased.