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I don't know about everyone else, but I had problems with the Steam version.
I pre-ordered it before the release date was changed. At that time, Might & Magic Heroes VI Shades of Darkness appeared in the list of games. A week after the date change, the game disappeared from the list. On the release day, the original Might & Magic Heroes VI appeared in the list. The second time I launched the game, it opened uPlay, where I entered the key that Steam offers when launching the game. Then the game launches through uPlay, with the uPlay username, not the Steam username. Although I have the collector's edition of the original linked to this Ubi account, the original game and everything that was on the account are missing in the game itself. There are only the two Shades of Darkness campaigns, the Prologue campaign (I haven't checked if the other campaigns open after completing it), and the pre-order bonuses. I'm currently reinstalling from the disc; let's see if it will work.