That is a very valid point reguarding the dark ages as well as other periods. My answer would be to pick one distict style of the time, arbitrarilly assign to it a nationality and cantury, then produce the a set as if it were the "uniform". Allowing of course, some variations for rank and things like that.
Sort of like doing a set of Romans exclusively in scale armour and calling then calling it the eastern legion, another in plate armour and calling them preatorians, still another in scale armour, segmented armour and so on.
As long as enough sets are done to cover the major/significant styles.