Units need their art design to tell a story. The easier it is to determine what a unit is, and what it does, based on how it looks, the better. In that case, let me propose some ideas. Hard edges versus smooth edges: IE, the difference between blocky brutalist russian construction, and flowing organic structures. My feeling is that the more hard edges used, the more focused on weaponry over support. A brutal assault unit is full of hard edges, but an engineer is a smooth, organic shape. Streamlining versus Blockiness: This is all about speed. A slim, aerodynamic profile- Even on a land unit- speaks of greater speed, whereas a blockier structure brings up thoughts of greater resistance. Size: I feel this correlates best to the resilience of a unit- With a few exceptions, the larger a unit is, the more capable it should be of surviving attacks. This can include vertical space, as well- Something that occupies a significant amount of vertical space may be more capable of surviving attacks. Just some general thoughts. Quick, instinctive understanding of your forces and those of your enemies is always nice.
On the other hand, the units are robots, so they'd probably fall moer on the hard edges side of the spectrum, and so there'll probably be some other method of clarifying the units role at a glance. I just don't think organic curves and soft edges would look right on a robot, especially given the art direction shown on the kickstarter.
Well, the kickstarter's art direction is fully evocative of Total Annihilation- Not at all a bad thing- Where all the units were very blocky. SC played around a lot with this, with the variation in hard edges, ranging from the extremely sharp and pointy Cybrans, to the smooth and alien Seraphim. Smooth edges can work on robots, but I'm not completely attached to it- It's just an obvious, easy art design choice to make that can emphasize a certain role.