Is Conflux a broken castle?

At the same time, I don't understand why they would prohibit upgrading the first, sixth, and seventh-level creatures of the Conflux. It wouldn't be that overpowered.

Buffed fairies with retaliation on day 1...
Imagine an army of 100 fairies with retaliation... they'll take down even stacks of Gorgons with ease!!!!
Mind Flayers - the same issue...
Phoenixes - no comments... due to their growth, their numbers are limitless, and the only way to balance the odds is not to upgrade them. ;)