But “I’m your father” works better because it happens at the middle part of a trilogy, and near the ending. This means the film (only considering Empire, not ANH) has dedicated runtime to set this up or foreshadow this in a way (e.g. Luke’s vision in the cave) and the twist accomplishes its goal as a climax and as a cliffhanger ending to get audiences excited for part III.
Palpatine’s return doesn’t land as well because it’s just dropped on the audience and expects the viewer to just get along with it, and the rushed pacing of TROS doesn’t make matters better. Even if it wasn’t spoiled in marketing, Palpatine’s comeback is stated right away in the opening crawl, meaning the audience is deprived of the chance to experience the revelation along with the characters. Seeing Luke’s reaction to his parentage reveal, coming from Vader himself, helps to set the mood of the scene, and we follow Luke pondering on this new knowledge the entire remainder of the film. He even acts as an audience surrogate by explicitly wondering why Ben Kenobi didn’t tell him the truth. Instead when Kylo lands on Exegol we are already expecting to see Palpatine, and it’s only the second scene. Of course it hasn’t been revealed how he survived or what his plans are yet, but since the revelation of his return is mentioned in passing as if it were an afterthought, it carries less weight than it should. It doesn’t help that the Palpatine retcon feels more random than the Vader retcon because it happens in the last film. There won’t be a direct sequel or an interquel trilogy to explain it (as far as we know the Skywalker saga is done). For comparison, a good chunk of Return of the Jedi is dedicated to confirm the plot twist (many viewers believed Vader was lying) and further develop the relationship between Vader and Luke and how it feeds Vader’s inner conflict. TROS is so rushed that the importance of such an array of reveals (Palpatine is back, and Rey is his granddaughter) doesn’t get enough time to be dwelled on or let the audience come to terms with it.