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Come on. This isn't math, where everything has to be precise. It's a philosophical question. Some people are fine with upgrading Pit Fiends and Cows, but I'm not.
Specifically regarding Bulls – do you upgrade them often? I practically don't build them, except in desperation, if I don't have enough money for Hydras. Therefore, I don't see anything wrong with upgrading them quickly – it balances things out.
The Pit Fiends have already been discussed, and arguments from both sides have been made. Which, as follows from the 1st paragraph, is a sign of balance.
By the way, Samurai and Redmi are playing on this topic right now.