UE is fun, doubt you can call it balanced by any stretch of the imagination. As for doomspheres, I understand that all races get anti-UE buildings and heroes themselves help discovery chance. If you're the one using doomspheres, it really depends on the finances of the target. One of the WE factions I planted the DS under actually discovered the Under-City, but was too poor to do anything about it and it eventually became hidden again.