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Champions? Why replace fast Angels with a large but fragile stack?
Besides their speed, Angels give a hero a second chance or replenish their losses in battles with neutral creatures, and they also perform very well under buffs, unlike Champions. Regeneration and the Guardian Angel ability are more effective when the stack has a larger health pool. But Champions will attack a more or less strong and resilient enemy unit, and then they can be defeated, and no speed boost will save them. Moreover, the speed boost is only effective on the first turn; after that, the Champions simply won't be allowed to gain momentum.
The same applies to Hydras, unfortunately. The unit is very strong, and it can also be buffed, but when the choice is between them and Champions, well, the Hydra doesn't stand a chance. In Might and Magic IV, speed is one of the most important factors for victory. With Champions, the army will have very high speed (and no other monsters are needed in the army). Also, Champions are good in battle because they can quickly eliminate enemy mages and archers when the heroes are still weak (battles with neutral creatures), while the Hydra also needs to be buffed, which consumes mana that the Chaos Knight needs to conserve, otherwise it will run out at the most crucial moment.