The hetaroi always struck me as a little odd. Zeus has inherent infantry bonuses, and access to many minor gods with infantry bonuses, thus his special infantry are a lot like beefed up hoplites, albeit less population efficient, but incredibly effective against non-Greek units (they do OK against hoplites in all the tests I've seen though).
Similarly gastrophetes benefit from Hades' archer centric bonuses, and act a little like a cross between beefed up archers and mini-siege weapons. They're expensive, but as everyone points out, incredibly effective provided you keep those pesky enemy cavalry away.
The hetaroi's stats are actually lower than those of standard cavalry, which makes no sense to begin with, it's a special unit, and even less sense because the in-game text describes them as elite hippikons. Shouldn't they outstrip hippikons then? Even with all the Poseidon-related cavalry bonuses, hetaroi are useful in such a limited context that they're hardly worth building. Kind of like pheonixes, the idea of a special building killer sounded cool, but in practice their usefulness against anything but buildings is so low they're hardly worth consideration. They're not totally useless, but you're better off just with hippikons, since you're Poseidon anyway and your cavalry's better.
Try a Poseidon v Poseidon game. If you even do get a chance to sneak your heteroi into the enemy base, they get killed by all the militia they generate. It's truly pathetic.
Given the effectiveness of gastrophetes and myrmidons, and the general ineffectiveness of hetaroi, some rebalancing (like actually making them better than hippikons) may be in order.