The concern here is that Rise of Skywalker may prove no more successful than Dark Empire in terms of justifying Palpatine's return. It makes the entire ending of Return of the Jedi ring a little more hollow. Dark Empire doesn't exactly reflect well on Luke, either. To learn that Vader only delayed Palpatine's plans lessens the impact of that scene. The scene where Vader finally chooses his son over his master and his own thirst for power is one of the most powerful moments in the franchise. Bringing back the Emperor cheapens Darth Vader's redemption and ultimate sacrifice in Return of the Jedi.
Above all, Dark Empire's core flaw is that it never truly justifies the notion of resurrecting Palpatine. However, these stories haven't aged particularly well. At the time, new Star Wars content of any sort was something of a novelty, so Dark Empire and its sequels struck a chord with fans.
