Demigod roles should be equally balanced.
The "Tragedy of the Commons" and the real reason the UB is OP...
There are five roles a demigod can take in game: anti-demigod, anti-creep, anti-tower, flag capper, and support. Unfortunately demigod-killing is far more rewarding in terms of total XP/gold than any of the other roles in the game. Ideally skillful play in any role should be equally rewarded. If you spent 2 minutes being a perfectly efficient flag capper or tower destroyer you should reap an equal benefit to two minutes of efficient demigod killing.
The UB is actually well balanced vs. the other demigods in a fair fight. The problem is that the UB benefits from a snowball effect of increasing power. He is the best at killing enemy demigods so he gets more gold, therefore more items, therefore even more demigod kills. It's a positive feedback loop. If you balance all the in-game roles as I illustrate above the Unclean Beast will lose his comparative advantage and be truly well balanced. Cutting the gold reward for a demigod kill in half should be enough to acomplish this.
This game currently suffers by being a perfect example of the "Tragedy of the Commons". No individual player will spend his resources (time, gold) on an action that benefits all other players on his team as equally as himself when he could spend his resources on an action that benefits himself massively by comparison: Demigod killing. Why spend valuable gold on citadel upgrades when you could spend that gold on an item to get a couple of extra demigod kills instead? Why waste time capping a flag when you could be plugging the Rook?
One solution is to make it so that purchasing a Citadel upgrade rewards the individual player as well as the team. Each Citadel upgrade should grant XP to the player who purchases it. That way spending your own gold benefits you as well as the team and the "Tragedy of the Commons" is eliminated.