Epic War Age (EWA) - Adapted Mod Collection

I can't understand what the difference is between MegaAI and Eternal AI?
Is there any point in enabling both of them? Do they work well together?

Yes, they have been made compatible. Differences:

MegaAI This mod enhances the artificial intelligence in Heroes 5. The main difference is that the AI now more often chooses skills that are useful for its race and the skills that it is good at using (and more often simply passive ones, for example: you can now encounter Retribution with Elves and Knights, and the Necromancer likes to take Fire Warriors… Also, the AI now likes to visit the library, hires only its own heroes (if there is a choice in the tavern), and so on… At the same time, the mod does not affect the game's difficulty settings in any way; it only improves the enemy heroes.

Eternal AI (Eternal Essence)
Allows for faster turns on medium, large, and huge maps + high intelligence: the AI calculates the development strategy several moves ahead, which significantly increases its intelligence!
+ the mod adds fine-tuning settings:
Lookahead Depth - Depth of turn calculation
Neutral Champions - Neutral armies
AI rush discount - Reduction of AI aggression
Artefact Guards - Artefact guardians (creatures guarding artifacts receive their bonuses)
Taxes - Taxes
Eight Skills - Eight skills