Is Conflux a broken castle?

Something is unclear. Is Conflux really the strongest Castle in Heroes? I've always considered it the weakest; almost all of its units are inferior to those of the same level. I've tested this myself. However, immunity to magic is a definite advantage, which might be why it's considered strong. But the unit growth rate definitely doesn't pay off.

What do you think about the second-level units from this Castle? They have 25 health points, and if you upgrade them... it will hurt. I believe that the second-level Elementals are the strongest among the second-level units. In my opinion, once I found a hero from this Castle in the tavern and decided to take him for an experiment. The experiment was successful: 25 Imps + 6 Elementals + a Magic Arrow shooter (with a magic skill of 4 or 5 at that time... I don't remember) easily defeated 150 upgraded Barbarian level 1 units. So, there you have it – a decent Castle.

Added 6 minutes later

You probably forgot that in Necropolis, the units are much stronger than the units from Conflux. Let's take Skeletons and Imps as an example. Skeletons have a growth rate of 25 per week, while Imps have 50. If you put them against each other (50 vs. 25), the Skeletons will easily destroy the Imps. Skeletons have 10 health, while Imps have 4.

But if you upgrade both, your 25 Skeletons will fall, while the number of Imps will remain at 50. Upgraded Imps are unmatched among the first-level units. The only thing that can destroy them is gold (Gremlins, level 1 Tower, where Titans are). But even then, I'm not sure... you can just experiment with this.