> I think its because historically mercenaries would fight less strenuously, and break at the first sign of real trouble as dead men can't collect their pay.
That's debatable. Swiss mercenary pikemen enjoyed two centuries of well-earned respect for their fighting abilities, for instance. Or you can take the Varangian Guard of the Byzantine emperors, or the Gallowglass mercenaries. The issue was more often that many of them would not hesitate to change sides if it was seen as more profitable.
Indeed, when Alexios Comnenus needed to bribe his way into Byzantium, he didn't even bother trying the Varangians, figuring that it was pointless. And the Swiss were known for loyalty.
Yes, I was more thinking of the European Renaissance period, where many armies incorporated large mercenary contingents which some of them, like the Landsknechts, happily changing employers. Italian Condotierri were also notorious for this behaviour.
Though it should be said that employers were not necessarily more trustworthy, and could be prone to "forget" to pay the mercenaries they had hired once they had no more need for them.
That's debatable. Swiss mercenary pikemen enjoyed two centuries of well-earned respect for their fighting abilities, for instance. Or you can take the Varangian Guard of the Byzantine emperors, or the Gallowglass mercenaries. The issue was more often that many of them would not hesitate to change sides if it was seen as more profitable.