it's a hell of a lot weaker stat-wise than a stegadon. Admittadly, still weird to have it that much cheaper given how an EoTG can cause a cascading effect.
They are identical in every way except for what's on top of them. Guess its a matter of is the Cosmic Engine rule equivalent to Alpha/Sunfire/Skystreak.
EoTG horns: 4/3+/3+/-1/2 Stegadon horns: 3/3+/3+/-3/2 EoTG stomps: 3d6/4+/4+/-/1 Stegadon stomps: 3d6/4+/3+/-/1 The stegadon is mildly better statwise. Then there's the alpha. And lastly there's the randomness of the EoTG. All of which are points in favor of the stegadon.
The rend -3 on the Stegadon horns is huge for me. When fighting Elite armies that makes a big difference.
I wouldn't be summoning Engines with conjuration. They won't activate immediately, which runs the risk of them getting blown away by the enemy and wasting your investment. Three Salamanders/Razordons cost the same and can start shooting immediately, so even if they die straight away, still value. It's why I also think besides the Eternity Warden or Astrolith, you shouldn't summon any of the utility heroes either. I would also favour shooty units in general, as without Cogs in speed mode, it's quite likely you won't make charges with any of the summons the turn they arrive (Troglodon can maybe do it alone if he kills something with the spit). Yeah Steg seems like a solid summon choice. I'm considering it over a Carno or Bastie at 24, simply because the flamethrowers look hilarious against enemy chaff walls. It's a bit weird that it has -3 on the horns but the Dread Saurian doesn't has to make do with -1 jaws and -2 claws.