Another major plank of Machiavelli's argument against hiring mercenaries went along these lines: either the captain is an effective leader, or he is not. If he is not, then what use is he to you? But if he is, you can be sure he'll try to wrest your power from you. Either way you are better off without hired soldiers.

And as Brandy Blue stated:

its not hard to develop a custom of keeping the casualties low
that's another charge that Machiavelli levelled at them - that they liked to make a great show of fighting a battle, but didn't really do much. Which meant of course that faced with any army from outside the "Italian way of doing things" they were pretty useless