Well, personally I am not so sure about stygia. They do have immortal sacreds, that's true. So does nehekhara, albeit you have to summon them. Still I'm not sure that's a big problem. And the freespawn are particularly bad freespawn imo. They do have some pretty nasty summons. But with how high they are, I actually don't think there's a big problem with them. So as far as I'm concerned, you can play them. I don't think they're in the same league as, say, ashdod. As long as we avoid stupid overpowered, I'm fine with it (besides, my nation oozes priests. I'm not afraid
).
With seraphim, I completely agree with you, they'd need some serious nerfing to be very fair. Some of those angels are just too much, what with three magic paths at level 4 and no handicaps.
And also, as I said you can play mod nations from any era. So don't feel constrained to just pick ma nations.