Total War's model fails entirely when we hit Modern armies, that don't rely on tight formations, but rather looser/skirmisher formations. It starts breaking down when we hit Industrial Age armies, because the ranged power creep is in much higher steps than the melee power creep that is, one side that has Fire by Rank and Platoon fire implemented becomes an order of magnitude stronger than a basic army without Fire by Rank. One can see in UG:CW where melee blobs start dominating because the computer can technically use the entire brigade to fight, whereas in Total War, your melee frontage is still limited by how many ways you're in contact with the other squad. Total War's combat model works a lot better for the classical/medieval armies, by virtue of the necessity of infantry squares. The biggest thing I would say that has huge difference flavors between the various franchises: