It being built on promise of being similar to IE games I think it's this biggest problem.
I think the whole concept of a game being a "spiritual successor" to some other game(s) should be abolished and never ever raised by any developer. The moment a developer slaps that "spiritual successor" tag on their game they are setting themselves up for bitter criticism and possible failure of their project because it is impossible for anyone to truly satisfy the nostalgia-driven feelings of fans of the original game(s). If the fans themselves want to make connections to some older game, let them do so. But the developer should strictly stay away from making any such connection claims.
If you are making a game that is specifically conceived as a callback to a particular game and need to encourage backers on Kickstarter, I can’t imagine not mentioning that progenitor game.
But as @Wormerine has pointed out, and with which I concur, it ended up hurting PoE2. Badly. Why? Because PoE1 was so heavily hyped as THE spiritual successor to the IE games that when hardcore fans of the IE games played PoE1 many of them came away disappointed that the game was not EXACTLY like the old IE games they fondly recalled. And that's why many people who bought and played PoE1 did not bother trying PoE2 (a shame, since 2 is a better game than 1 in many ways).
By making PoE1 as a spiritual successor to the IE games, Obsidian limited itself creatively to trying to mimic those old games and not doing anything new or innovative for fear it would alienate fans of those old IE games. And yet despite this, they still ended up disappointing many of those IE games fans anyway. So they shot themselves in the foot not once but twice! Firstly, the lack of newness and innovation in the game meant critics were not as impressed as they could have been, and neither were new gamers who had never played the old IE games. And secondly, even in the face of Obsidian bending over backwards trying to create a game that mimicked the old IE games, fans of those old games ended up disappointed.