Cerebus : 1250 Wood, 1500 Stone, 3000 Coin = 5750 total resources. (also has a 230 favor cost per unit)
Swordsman (30 population): 2850 Wood, 2550 Coin = 5400 total resources.
Hoplites (30 population): 2250 Stone, 4500 Coin = 6750 total resources.
Now you can argue all day about the value of 240 favor, but in a comparison of normal resources, the Cerebus is quite a good deal. It does the job defensively of both (and then some) at the same time, with only the relative cost of one.
Cerebus: Ranged def = 1575, Blunt Def = 825
Swords(30): Ranged def = 900
Hoplites(30): Blunt Def = 540
The Cerebus also saves you in total build time, taking 30 min with lvl 10 Barracks, while 30 swords takes nearly 2 hours. (World 3, speed 4)
The Erinys on the other hand is quite poor in almost all aspects. I can't find a redeeming factor except a small gain in total build time vs. 55 Slingers. Those same Slingers save you in resource costs and do more total ranged damage. The defensive numbers aren't even worth bringing up. If these are to be an offensive unit, then they need something added to make them more useful in some way.
Give them flight perhaps, or up the damage, reduce their population used....something. If these are the final design going live, I believe the Erinys will simply go unused. Seems a wasted effort to even add them.