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Well, I'll take the initiative then, who's left?
Ballerinas: In principle, they're a decent unit, although I don't really like them; I'll explain why. They're not very useful. In the early game, everything depends on the Dryads, and the Ballerinas can only be used to distract enemy monsters and occasionally finish them off. As for the upgrades, the old upgrade is mediocre. The fact that it attacks all targets is, of course, good, but it's weak, and it also has retaliation. The problem is that the more units they attack, the more they become a target themselves, and you end up having to protect them and only attack units that don't have retaliation, because they're too weak. However, I like the new upgrade; you can throw these guys into the front lines without worrying too much. In fact, they control all the critical points on par with the Unicorn, but the problem is the same: they're very fragile.
Zombies, they're zombies, nothing special. They're just meat that allows the Necromancer to last longer in battle. In the early game, I take the first upgrade, and towards the end, the second.
Spearmen are good units, especially in terms of their usefulness to the army. The regular ones have too few shots, but they can also hold their own in melee combat. The first upgrade is simply excellent. I don't know about everyone else, but for me, the Crippling attack triggers every other shot, which is very good for the slow and weak-initiative Orcish army. The second upgrade is just complete nonsense; no sane developer would come up with such a thing. The only use for the harpoon, as far as I remember Wolf said (or maybe it wasn't him), is to pull enemies into the Fire Wall. This is a rather dubious (nonsensical) ability compared to the Crippling attack of the first upgrade.
Centaurs are not bad, at least because they are the only ranged unit in the Orc army, and they are good ranged units. They take out enemy ranged units in two hits, plus they have Blood Fury, which makes them really strong ranged units. In my opinion, the second upgrade is better than the first, at least towards the end, when there's not much space. These guys (the horses) can hold their own in melee combat.