Is Conflux a broken castle?

I don't really see anything unfair about the Conflux. The only thing I disagree with is the increase in Phoenixes. There are too many of them. Yes, they are weak, but 4 per week is too much. In my opinion, 3 would be enough, and even that might be too much. 2 would also be a lot.
At the same time, I don't understand why upgrading the first, sixth, and seventh-level creatures of the Conflux should be prohibited. It wouldn't be that unfair if their upgrade was allowed. Or would it?