The biggest problem is that most weapons for empire and kingdom are same (same stats), but the way they are researched is completely different.
Kingdom gives better and better weapons every few techs, so you would expect those weapons to outperform/replace previous ones, and that getting all those extra techs means something.
On the other hand, empire lumps one set of weapons all at once (for example same tech gives mace and all 3 warhammers!!!), so you would expect that better weapons are stronger, and weaker ones are still useful by being more cost-effective (otherwise why bother giving them to the faction).
These two goals collide with each other, and make proper balancing difficult.