In your opinion, which faction is the strongest in Heroes 5? Let's discuss the advantages and disadvantages of each faction.

So far, the ones protecting their races are: Necromancer, mages, all elves and humans; only the imps are missing.

It's just that few people play them.
But I can protect them too. They have a wonderful ability - Call of Urgash. It helps to immediately deliver units from Sheol. It harmonizes very well with the Wide Gate and Quick Opening abilities. The Mages Guild is focused on chaos and darkness. The hero's main characteristics are attack and knowledge. They can overwhelm the enemy with their spells. Since I played as Inferno, at level 23 the hero has 190 mana. During a siege, they can send summoned troops through the walls. This helps a lot. Archdevils resurrect Cave Lords. Lords wield the scripturing sword, so even 1 Cave Lord can take down a phoenix with 1500 HP or more