1st : You can not have 100%
historical authenticity or 100% realism in a game. Why? Simply because there are some features AI can not hundle that humans can.
Example: There is no unit in a game that can use entirely diferent weapons the same time. I explain. You can not have a unit that uses archers, spearmen and slingers together. This may look silly to some people eyes but its essential to
authenticityand realism. The 1st compromise is the devide units in to imaginary separations like swordmen, spearmen etc...
We can have models of warriors that can carry multiple weapons but game engines have serius limmitations.
For example in M2TW engine that had more complicated animations than Rome II (and seems Attila also) when we tried to create Spearmen that could be swordmen as well we encountered a problem on the proper use of spear or sword. This looked quite natural in cavalry units but prevented infantry units to behave in battle properly.
Having this in mind we see that battle realism will be always far from accurate.
Ofcourse modders (developers do not care about such issues), sometimes find compromissing solutions in many cases.
I am not sure if in the new kind od game engine that Rome II and Attila work, allows some animation combinations like in M2TW. But there were very succesfull weapon combinations in the same unit like two handed axes and suitch to sword use at will!
Another issue is the personal sense of each one of the players about what is historical authentic and realistic. Lets supose that you as a modder find a way to portay as many as possible , historical units in the game. Many people will doupt the authenticity of your creations even when you will provide everything to prove your work right!
Do you believe that players would like to exclude (in Attila as example) siege equipment from all barbarian factions as realy was in history?
Do you believe that all those that praise CA for the famous siege escalation would like to have bloody and almost impossible attacks of fortified settlements?
Do you beleive that most of the players here would like to see Franks without horsemen and all factions without pikemen as realy was in history?
We can not have 100% accurate games. But this is not an excuse to have imaginary based "historical" games too!
Game's limmitations can be also a tool of 90% accurate presentation of the warriors of that time.
M2TW modding creations lead the way.... Lets suppose that you study history and you encounter 2 units of similar nature.
Lets supose that those ywo infantry units used both swords and spears (as naturaly human warriors used both). The options are:
Create two versions of each unit and name them:
Unit 1 swordmen, Unit 1 spearmen, Unit 2 swordmen and Unit 2 spearmen. That consumes unit pool places without any clear benefit.
The 2nd option is to create the unit 1 as swordmen and the unit 2 as spearmen and in their describe texts you can explain why you did that separations and how were these units in reality.
Unforthunatly , realism is so borring for many players that create unit rosters in imaginary basis! You see steppe pikemen, steppe macemen, frank cataphracts etc...