One thing I haven't seen touched upon much is how difficult it is to maintain control/discipline over your forces depending on where they are from, which can have varying levels of bad consequences.
Militia seem prone to getting extremely... passionate, or vindictive, especially if your being invaded and/or the militiamen perceive that they are victims of something. During the very first battle of the American Revolution, already you had militiamen scalping redcoats, and militiamen travelling from dozens of miles from the surrounding countryside just to get a chance to shoot at the fleeing redcoats. Were it not for the timely arrival of reinforcements from Boston, the British forces that went to Concord would have never made it back alive to Boston (and even then, only a fraction of the initial force made it back alive). You also have instances where militia/patriots who perceive that someone isn't patriotic enough will brutally punish said person by burning their house down, or "pressuring" that person to leave the country (which in turn only encouraged the "punished" individuals to join the Loyalists).
Mercenaries are obvious enough. They follow the source of the most coin. If they aren't paid on time... or think they will get more money by going rogue, they will go rogue. IIRC during the Spanish occupation/invasion of the Netherlands, the Spanish missed one of their payments to their mercenary forces. Said mercenary forces then proceeded to brutally sack a nearby wealthy city, which galvanized Dutch resistance against the Spanish.
Hiring/allying with foreign tribes or clans that do not share your culture to fight for you seems to be highly volatile. During the Revolutionary War, an Indian tribe that was allied with the British scalped a few civilians during one raid, which fed the Revolutionaries' well oiled propaganda machine. "The king is hiring savages to butcher his own people!". The Indian tribes hired by/allied with the British during the Revolutionary War were either not given proper instructions (perhaps the British liaison overlooked this, or something wasn't communicated properly), or the Indian tribes were bloodthirsty or ignorant enough to not care.