Will it be a thing?
Friendly fire has been a problem for a long time, and still is today. In those giant mosh-pits, that is major warfare, you how will you recognize friend from foe?
I would personally like to see a system, that doesn’t care what team you work for. If you get hit hard enough, you die. But only if the AI is geared up for it. The AI should be aware of it’s teammates, and not get in the way of either PC or NPC.
It could be harder to kill an ally then an enemy, and when they were back in town (if they didn’t die) they would shout at him “Train you aim!” or “Could you imagine that SOMEONE had a little to much ale before battle?” or something on those lines 
You’re mainly going to be fighting in the company of a lord and his army in these large scale major battles, meaning that your team will have a distinct uniform so you cal tell friend from foe, also friendly fire might happen from the side of the bot but only if the AI because of bad skills maybe or bad training didn’t hit in the correct way and chopped your head of instead, I guess the chance is there but it is low.
Archers were not really often ordered to fire volleys during infantry engagements. Once infantry (or cavalry) were fighting in a field, the risk of friendly fire was too great and would only be commanded in a case of desperation, if the commanders infantry was losing to thin them out, then maybe run them over with cavalry.
a hit is a hit. no sword or arrowhead cares about who it is hitting.
so, if it should be a realistic game, it should make no difference either.
