Aww, when I saw "hoard" I was hoping for dragons!
But still, Horde and Lich. That gives us a solid foundation of numbers with their primary weakness, well, Lich can only be in one place at a time, but somewhat annulled. But more importantly it gives us the lich from whom we can learn mental faculties with no biological basis and magic, combined with unsoulled who offer example with which to extend into formless beings and also provide mindlessness which, while not always ideal, can be a massive boon against all sorts of things like intimidation, propaganda, a wall that can easily be passed just by piling the corpses of our own forces into a ramp and still swarming over it because we just have that many of them... Hrmmm, an even cheaper version of soulless based upon the theory that we currently have adult humanoids as a base, and that humanoids tend to become more defined with age, and that they are cheap because they lack definition... If we could then revise our tacticsto recognise unit types to send in the cheapest first...
Oh, right, advantages. Well shadow crystals sound slightly better than fogstone, what with crystals being typically lighter, but fogstone sounds more utilitarian, with it being used as the haft on a spear and thus probably focused more on sturdiness than the shadow crystals which are only tested in imp-sized daggers. Otherwise they are both apparently semiclear mineral items that can be used to build a sharp edge and a handle. They are also the only group which starts with a tactic, which is typically a difficult thing to come up with a good design for. The magic wands of the imps would be very nice, but we do already have an "in" with magic if we choose the lich and imps are a bit too heavily attributed for my tastes. I like the idea of dominating the fire plane, but then what? and the orc additions seem kind of weak. I mean, no doubt orcs are good enough to make up for it, so long as the enemy don't use silver, but we would pretty much need to replace the rest of the set with armour that is tougher than orc hides and weapons that are harder than orc faces, and we don't really get anything out of the base models so that ends up being pure wastage.
I do really like starting with a cheap unit too, cheap tends to be difficult to do comfortably without losing all their defining characteristics, which is too much tragedy for most designers to handle. Meanwhile expensive units are usually pretty easy. Like, you'know, just reanimate a cheap skeleton using the lich as a working example of animate corpses, then cover the skeleton in soulless flesh bound to the skeleton's will and teach it a simple bolt-of-death magic. Then you have nice elite mage units to sneak amongst your ocean of bodies to throw some damage around. Or you can pack a hundred soulless into a single deathball to roll across the battlefield sowing chaos and being largely immune to the area effects that only affect your weak units. Or create a soulless that can consume the substance of its enemies in order to gain the features that it lacks, thus creating an infiltrator unit, just as soon as you can give them acting lessons. Or little rolling clusters of skulls to act as generals leading our armies in the absence of the lich's presence. Or if we can't afford the full lich, then just use its severed skull as a really irritated magic wand. Anyone can come up with imps that sh have magic wands for eyes that ignite everything they look at, or orcs with cybernetic implants, but the cheap imp or cheap orc are just really difficult to design without having stupid imps with no magic or tiny orcs with no strength, neither of which work particularly well...