These types of games typically only let you bring in a set number of groups of units. My only concern with being able to make customizable units is that it would exacerbate some of the unbalancing problems that occurred in MOM. For instance, magic immune, first strike flying paladins were basically unstoppable in large numbers. Add the trolls regeneration ability and they would be unstoppable. No force could elimiate them completely given the unit number limitations, they would regenerate, and then take out the next army, etc. Not that these specific abilities will be available in Elemental, but I just think that some thought should be put in to limiting exploiting mechanics.
Well you just cited the source of this problem in the first place - that lots of these games limit the number of units per army to something ridiculous like ~8. Stardock has already said that they want the tactical combat to provide epic battles of massive proportions, and for that to be possible it seems they will not be following in those footsteps.
In fact, in the "8 units per army" model, quality is the only important factor. There is no way to overwhelm superior quality troops with poor troops, because you can never bring more of your crappy troops to the table than your opponent can bring of his flying paladins or whatever. The instant you take away the "8 units per army" barrier, the problem largely solves itself. All of a sudden I can now bring an equal cost of peasants to fight your paladins, and 100 peasants per flying paladin might just be able to overwhelm them with shear numbers. Additionally, if combat is at all TW-like, then the fact that your paladins can only attack one peasant at a time will help balance it all out as well. In MoM the paladin could wipe out a squad of, say, 4 units in one turn with no retaliation. In TW-like combat, your paladin might be able to kill one or two peasants before he's surrounded by them and taking hits from them. He doesn't get the opportunity to wipe out an entire squad of peasants with no retaliation, because he can only attack one peasant at a time, out of a squad of many.