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

The mages of Haven are quite powerful. I never use them to cast spells – it doesn't really give much of an advantage. Endurance and other stats don't provide a significant bonus in combat (though someone might disagree). I usually use them to attack.
Dwarves are still better than humans, and the Rune Priests are superior. However, the Dark Elves are once again making their presence felt. With their overwhelming magic, the dwarves will have a tough time.
Generally, I've found that Necropolis is best suited for large maps, as I've mentioned before.
Overall, I've come to the conclusion that Haven is the most vulnerable faction (in my opinion), but if anyone disagrees, they should provide convincing arguments.