Lorewise it doesn't even make much sense to make it a race. It's explicitly said the Advisor's tome is what allows whatever LL he picks to cross into the Realm of Chaos. Kairos Fateweaver himself, basically the most magically knowledgeable being in the setting aside from his boss, is at his wit's end trying to find an alternative... but in-game everyone from Kislev to the Ogres knows about Ursun and somehow manages to cross no problem. Whereas in the Vortex everyone could see the comet and all 4 participating races are noted as magically adept.
There were better ways to put ramping pressure on the player than to make it a race that can feel simply unfair if the RNG screws you and leaves you with little margin for error in a game that's already harder than the second one. Such as diplomatic maluses with the factions of whichever God you nick a soul from; I don't see Ku'gath appreciating you trashing grandpa's realm, for example.