I would argue that Infantry, or rather 'Biological unit that is a representative of the Faction', is a specific enough subgroup that is shouldn't be a great hassle. Obviously, it depends on the particular Secret Tech, but Promethean only has one. Xenoplague might have one, but feels like zero(unless a mutated subject of the faction is their T2). Psynumbra is right along there with zero to one most likely. "-sis" seems like they might be Cyborgs of some kind(at least the flavor of the Secret Tech suggests it), and if so are likely unaffiliated to Faction appearance(special exception, but I feel if they are Cyborg-type units, then perhaps they should adopt Assembly appearances when played by Assembly). VoidTech and Celestian are up in the air, but in general only the T2 slot sems likely to show Faction representation in most cases, as humanoid T3 units seem rare so far. With Valkyrie being an exception(and a squishy one at that!), and probably a beast-riding Amazon, or simply some sort of Battle Mistress that's on foot.
Anyway, if we make the reasonable presumption that, on average, only one unit per Secret Tech is in representation of Faction selection(potentially some with zero, maybe some with two), then that's only six extra units to design, with minor tweaks between them to adjust for faction aesthetics and body types. Thirty-six total across all combinations, with many being relatively minor tweaks. In particular, Vanguard, Amazon and Syndicate appear to share a similar body type. Possibly Assembly, too, for their less monstrous units, from what we've seen of their heroes.
Introduce a new Secret Tech, and that's another six units(or none, or twelve or even eighteen, technically), or introduce another Faction, and that's still only another six units(with relatively minor tweaks on the existing designs, depending on physiology of the introduced Faction) on top of the Faction's units. Sure, it gets a bit more complicated a couple expansions down the line, especially if they release multiple Secret Techs and/or Factions in a single Expansion(I would still think there might be at least one of each, though, alongside an NPC-Faction, and whatever new game mechanics would be released).
Golden Realms would be the equivalent of an extra Faction, counting the Halflings and Wild Magic mostly. Eternal Lords would be much larger, with two Factions, and a Secret Tech, more or less, worth of additions. Hopefully, we get expansions closer to Eternal Lords, but we'll see. I'm just hoping for a Faction and Secret Tech, at least, per expansion, if possible. Although I would also be interested in simply getting multiple new Secret Techs at once, along with whatever else comes with them. New Factions are more dynamic, though, for obvious reasons.
That said, I'll point out that the far and away most subscribed Mod for AoW 3 is the Racial Class unit reskin mod, by the lovely Tibbles(who also helped or developed Triumphant Heroes and Extra Support Units). Inevitably, if the Devs don't do it a Modder will, but I'm personally in favor of the visual Faction variation being built-in to the core game. I know, though, that mechanical variation outside of core Faction traits won't exist on the units, simply due to Unit Mods fulfilling that purpose, and am perfectly content with that.
