Problems with Heroes of Might and Magic IV.

I was wondering why many players dislike the fourth installment? After all, it has its own unique features, just like the other games in the series.

because many players judge games based on their first impression. It's like a stereotype; no logical explanation is needed.
So, someone sits down to play the fourth game after having a great experience with the first game they ever played, the third installment, and it doesn't work out... and everything seems wrong... and besides making the heroes "overpowered," they don't see any other options in the campaigns... so what happens? - It's an RPG. They delete the game, go to a forum, and trash the entire game.
And no one will listen that the fourth game is a much more complex and interesting strategy game than the previous installments, no one will listen to advice to "figure it out" first, play multiplayer, etc. No one needs that.