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

What are you talking about? Don't the people you know play? In fact, most people play on large maps for a grand endgame, to fully upgrade and utilize all possibilities... This is where the balance ends, and mages dominate.

Maybe somewhere they do, but not here... Large maps are interesting in single-player, but in multiplayer, fighting against another player is much more interesting than clearing out neutrals, so the pace is the most important thing. Only those who benefit from it prolong the game, but in that case, the opponent, on the contrary, tries to attack as soon as possible while they still have a good chance. It's a calculated move.
In general, I don't agree that the Wizards are the strongest... They are definitely the most unpredictable because they depend heavily on resources and what appears in the Guild of Mages (which allows you to develop any school). Once, I played with a friend, I was Necromancer, and he was Wizards. In the final, I was surprised because he had Darkness... And he did well, although I still managed to win thanks to my army. But another time, it might be extremely unlucky; it varies from game to game.