The situation is, in fact, slightly worse than we might think. A set of wargame rules is not a single model . We have models built in for all sorts of things, such as movement, shooting, close combat, morale and whatever else the designer thinks might be important. The way the model set is presented might, and usually is, different, but, nevertheless there is this set of models.
In fact, I think it might be argued that there are nested sets of models. For combat we have, perhaps implied, models for each different troop type against each other type. These may, indeed, sit within an overall combat model, but we could regard each combat type, by which I mean say, cavalry against cavalry, or infantry against artillery, as being a model in itself.
This might sound slightly alarming, but I do not think there is a particular cause for panic. In many areas of life assorted models are used quite happily up against each other. In physics, for example, we have wave-particle duality of quantum particles. This is, in fact, two models, that of an electron being a wave and that of an electron being a particle, which coexist quite happily alongside each other, even though the idea is counter-intuitive.
Even in less esoteric worlds of human endeavour we tend to have multiple models. I have two alarm clocks in the house, and they do the same job, but the user interface they present are different. The designers, presumably, had different ideas of how a person would come and set up an alarm clock. These interfaces are, of course, models, in that they attempt to form in my mind a mental map of how to set them.
The fact that one is more successful for me than another is not really here or there. The fact is that they are two models of a task that I would like to do.
So, two sets of wargame rules are, in essence, two different sets of models of some sort of battle reality. I think this has some implications for how much we like given sets of rules.
Firstly, there is the question of whether the individual models in a rule set fit together. If, say, the morale and command rules bear no relationship to each other, or contradict each other, then we might not like the rules. If my commander is within command radius and yet cannot encourage his men, we might look a little askance at the rules and declare the inconsistent. There has to be an internal coherence to the model set to make the rules work at all.
Secondly, the model set has to, in some way, correspond to our expectations of the real world the models are abstracted from. If, for example, our early WW2 Matildas are regularly taking out Tigers, while 88 mm shells bounce of the armour, we might suspect that the rule set was not a particularly accurate one. Similarly, if our Assyrians routed when a single Egyptian chariot appeared on the table, we might think that the rule model set needed some adjustment.
Thirdly, the way rule models sets are constructed is via the modern philosophical standby, reductionism. In my description above is reduced the battle to a set of component parts: cavalry against cavalry, tanks against tanks, or whatever. We then can model the individual bits, and build up our set of models to acquire an overall process of the battle. The battle, we imagine, is simply to be reduced to a set of interactions between these constituent parts, in more or less complex ways.
The question that arises is, of course, the one which arises for reductionism anyway. Does the sum of the parts actually give us the whole?
I have mentioned before the interesting fact the individuals behave differently in a crowd to when they are on their own. Of course, in military units that goes further. Individuals are trained to act in a different way within the unit. That is, after all, part of the point of having a unit at all. So we cannot, in principle, model a unit in the same way that we model a whole bunch of individuals. There is, in a unit, some emergent behaviour.
This is not, of course, fatal to our ability to wargame. In fact, given that we do not need to model the individual because of the emergent behaviour of the unit, it simplifies matters considerably. All we need to do is to successfully model that emergent behaviour.
The question is, then, do we need to go further. A set of units is, of course, a higher level unit, a division, or a corps, or even, depending on your scale of thinking (or the size of the historical prototype) an entire army. Do these objects exhibit emergent behaviour too, and can we, should we model it?
Now, many rule sets to at least pay lip service to army level emergent behaviour. In PM:SPQR (and, I think, PM: ECW, I must be more persuasive than I think) army morale is modelled. As a rule set they are not alone in having a model for emergent behaviour at the level of the whole force, but, given that there have been a fairly limited number of armies (as opposed to units) modelling the top level behaviour can be a little tricky.
Nevertheless, I believe that, historically, army commanders can affect the emergent behaviour of armies. Montgomery seems to have been able to do it. Wellington certainly seems to have thought he could do it. But it is not something that, it seems to me fits easily with our highly reductionist standard model sets for wargame rules.
Or, maybe, I am simply overemphasising something quite obvious again.