This is a very good point. The types of helmets used by different "nations" varied quite a bit. Not only that but also the type, shape and painting of shields also varied a fair bit.
"However, as we have mentioned before, we are a small studio and our art budget (and art time allocation) is not infinite. "
Making appropriate models for all different army lists would a costly endeavour even for bigger studios, to be honest. I mean the variety of different types of equipment was far greater than most people (including players) realise. People should just accept those unit models are just a symbolic representation of their troops. Period.