Yes, microing COH-style cover would be no good, good point. Retracted.
Differentiation: To be fair, my ideas are based upon untested preconceptions. I will need to stage mock battles to get a better idea of the situations in which each unit excels. If I put a group of archers against an equal sized group of Brutes, which will win? I have no idea. If you have a group of half arches, half brutes against a group of all brutes, which will win? I don't know.
My lack of knowledge and understanding aside, here are some ideas:
1. Brutes have 50% battle speed instead of 25% so they can clash faster, providing a buffer for ranged units.
2. Scouts (cannot remember the name, Aegis?) have no battle speed penalty, low hit points, a medium range low fire rate high damage weapon. They travel at the front of the group, but when fired upon they rush behind the front line of brutes, darting forward to snipe when they can fire again, then darting back behind the line.
3. T2 and T3 buffs and tactics mimic and support the role (Zeus provides a small speed and defense buff to brutes and fights on the front lines with the brutes, Artemis hangs back and provides a weapon range buff to archers, etc.) (I'm blanking on the actual unit names, I hope you get the gist anyway).
All these ideas would also serve to add a bit of dynamics to battles, all of it handled by the AI so no microing.
I think I recall in previous posts a dev saying they want to avoid rock-paper-scissors-like relationships. I could be wrong. I think it is an elegant mechanic, though, so some degree of RPS does make group composition relevant.
I will be sure to post more ideas if they come up.