I have not tested this, but it seems to me that by the time Ophanim is available, Merkavah is not far behind and it is very tempting to wait and get Chayot rather than spending the gems on just Ophanim unless Ashdod is in the mids of a life and death struggle and cannot wait.
Chayots are awesome, but they are not unique and so other nations can get it through wish. So it should not be too unbalancing.
I'm less bothered by Lords of Civ than Ophanim, since I have nothing to compare them with. They are summoned as SC's without any gear. Seraphs etc needs some equipment to be efficient.
The difference between a LoC and a seraph is not that big. Just a pair of wings
I think by the time you summon seraph, gears would be the least thing you would worry about. Ophan is great against smaller beings, but against any other SC, it is dead meat, I mean wheel.