I believe that imbalance has its advantages; on the contrary, I don't really like the simplistic balance in the third game. The ability to upgrade everyone is not interesting. If everyone can be upgraded, then what's the point? But in the second game, it was cool. Maybe not everyone could be upgraded, but dragons could be upgraded twice. And I think the upgrades in the second game were more logical. How can you upgrade Rocs or Hydras? They just reproduce, that's all. Maybe breed a new species? And in general, I don't even know why I like the second game more.
So, it turns out to be a kind of more complex balance.
It's no coincidence that the weakest Knight army has everything built up, while, for example, the Warlock has three upgrades for the Wizard and Sorceress, and the Warlock only has the 5th and 7th ranks.
Very good idea... although the developers may not have thought about it...