I think the biggest failing of TROS was bringing back Palpatine. Kylo was set up in TLJ to be the ultimate villain, and then he was just sidelined in favor of “rotting grandpa GLaDOS.”
I don’t think it’s wrong for Kylo to be redeemed, so I disagree with Dual of the Fates; but I also think he has to more than just redeem himself - he has to fail before he can be saved. TROS should have been the culmination of a really interesting negative character arc; and instead, he just spends the entire movie chasing after Rey. Then his mom dies, and he just…snaps out of it, I guess?
If I were writing the script, it would have focused on Kylo trying to be the Big, Bad, Supreme Leader, but failing at it. The galaxy, inspired by Luke, is revolting everywhere he looks, and the First Order is spread way too thin. The First Order Generals and Admirals are all starting to question his abilities to lead; Hux is planning a coup, and is waiting for the right time to strike. Kylo is stressing out. He was already somewhat unstable (as seen in TFA), but now he’s just going crazy. Any time someone questions his orders, he cuts them down. And the more he loses control, the more the First Order splinters, the stronger the Resistance grows. He’s the maker of his own downfall.
Because he was wrong to pursue this path. He was wrong to join the First Order. He was wrong to seek this power, power that he’s not equipped to wield. And, try as he might to ignore that fact, over the course of the film, he has to finally come to terms with it.
When he finally turns back to the light, he does it in order to save his mother (building off of that moment in TLJ). And then, he sacrifices himself not just to save one person, but to undo all of the damage that he’s done to the galaxy, to atone for his sins. His final act is to destroy what remains of the First Order at the cost of his own life. Leia survives, and goes on to rebuild the Republic (correctly, this time); and Ben is able to visit her as a Ghost.
I really don’t have any major issues with VII or VIII overall (even if they could be improved upon); but IX was the only film in the bunch to truly fail for me.
Palpatine needed to be brought back. By bringing him back this ties the entire Skywalker saga together and gives IX a sense of true finality; he also needed to be related to Rey, to reinforce her toxic core belief that she is worthless. It also pays off RotS, in which he was set up to have a power to keep himself alive in a way Plagueis couldn’t.
Regarding the “tying the Skywalker saga together” thing, Abrams said this:
…when you look at this as nine chapters of a story, perhaps the weirder thing would be if Palpatine didn’t return. You just look at what he talks about, who he is, how important he is, what the story is — strangely, his absence entirely from the third trilogy would be conspicuous.