Machiavelli wasn't wrong about mercs. To dismiss his advice because he didn't have much military experience is nothing short of foolishness and hubrys. The dude was a diplomat who witnessed first hand the conduct of mercenary companies in the italian wars. While it is true that not all merc companies switched sides all the time, they were generally less trustworthy than regular troops. Units like the Swiss Guard of the Pope were by no means the rule, as a matter of fact that's why they're remembered unlike many other companies. Also, the National Guard of France sided with the government, so I don't know where the betrayal is.