For supremacy, the lines between man and machine are blurry. They basically are machines.

If anything, you would think that Purity would use robots/drones and Supramcy would focus more on biological enhanced troops. Indeed I feel there should be a fourth Affinity and that would be Synergy. Supramcy would focus more on biological upgrades (KHAAAAN!) while Synergy would focus more on Cybernetics.

Purity would want to use robots to keep the all important baseline humans safe.

Also, from a symbolic stand-point, 4 is always better (Earth, Air, Fire, Water sort of thing) You get the more mechanical affinities (Purity & Synergy) along with the more biological ones (Harmony & Supremacy) each with their own focus.

@zArkham4269 As for missile rovers, one could classify some units as heavier targets in need of more firepower. The top tier UUs for example, and have the missile rovers gain strength against them.
Or perhaps something where the missile rovers damage doesn't drop as fast when it is below the targets strength. Allowing it to maintain more consistant damage even when out gunned. They would be the natural choice for defending against more advanced units. Maybe the pre-patch Battlesuits wouldn't have had to come down in strength so much if there was a counter to their high strength that wasn't also a level 6 unit.

@zArkham4269 It is hard to predict the future of technology. Look at old sci-fi. Always tons of spaceships, no internet.
The generic units seem to follow the clasic military unit types. The unique units got with a futuristic direction although more in flavor than function, given that we don't know for sure what function they will have in the future.
Supremacy does the all drone, all the time route. Purity showing an aversion to AI has classical sci-fi one man armies via mech suits and the super strong, super light materials in hover vehicles. Harmony using miasma is more in keeping with what we'd expect biowarfare to look like. While the units with the extra strength alone perk fit the one man (unit) army role better than purity.
If population itself was a resourse for the military then the difference between fielding robots, humans, or non-human organisms would be a factor.

Obviously one of the issues is we are currently moving away from a lot of weapon systems which use to be surpreme. Jet give way to drones, tanks to heavy infantry support vehicles and the like. Sci-Fi tends to go with super dense materials paired up with something akin to contra-gravity to bring tanks, jets and the like back.

But really, at the beginning, we're looking very much of an infantry based world. Of course the biggest issue I see is people are probably the most strategic asset of Planet. You'd expect more emphasis on drones, robots and Bolos/Ogres.

@zArkham4269 That is an interesting thought. In historical civ it made gameplay sense to keep the siege line of units especially since in vanilla there were no moddern ranged units and when they were introduced they were short range and more defensive.
Beyond Earth seems like an oportunity to break that since it doesn't start with catapults. And as has been noticed by just about everyone, the other ranged units work just fine against cities. So perhaps artillery should find a new role rather than try and fit the old one.

It's kind of funny how games continue to make missiles the equivalent of catapults. Missile Rovers should be better at attacking tanks and it should be infantry which should have bonuses vs. modern cities.

That being said, this is a good use of fixing something I've been noticing a lot lately.