Sacred Haven of Debauchery

Usually, not very good players (maybe noobs) play as those factions that are easiest to play.

Good players, on the other hand, play as those factions that allow them to win more easily, because they know the strengths and weaknesses of the game. And noobs are noobs because it's difficult for them at first, as they don't understand anything about the game. So, if you find it difficult to play, it means you're a noob. This completely contradicts the statements of Morning and WisTrit.

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It's easy for me to play as the undead; they have the fastest development, almost no losses, and a fast start! I just don't like playing as the others!

We're not just talking about heroes right now.

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But if you're already good at playing as one faction, and not so good at another, you can play as the second one to get more enjoyment from the challenge.

You can, I agree. But the question isn't about that. The question is that some are clearly better, and some are worse. It all depends on the game's balance.