Well, I don't know how survivable they are right now. You start with a settler, an adept, and an angel. You still can't build anything until Hunting, and can only build hunters till.... I think founding a religion is actually the next closest unit for them. Or if you have Horses nearby you can snag them and have some mounted units be your first possible buildable unit, maybe (not sure if they have Horseman blocked, I don't think so. Honestly confused why they don't have Hunters blocked, but have almost every other recon unit blocked). So while the starting Adept and Angel might be fairly strong, if you lose either one due to random odds, you are mildly hosed.
Anyway, both Mercurian and Infernal are still MEANT to only enter the game at Mercurian Gate / Infernal Pact. But I can understand wanting to use them earlier, so the option is just a bit more balanced now (I hope)