At the end of She-Ra a few plot points, both minor and major, are left unresolved but it didn't brother me because the show still resolved its most important arcs. Catra and Adora admitted their love for each other, Glimmer was reunited with her father, Catra made peace with those she hurt and even Shadow Weaver managed to have some sense of resolution with her "daughters". So, when all was said and done fans felt satisfied because it resolved what we were most interested in.
By contrast, Rise of Skywalker resolved nothing, or what resolutions it did give weren't exactly satisfying. When you break it down the only arcs that are resolved in ROS is Poe Ready to lead the rebellion and can Rey connect with the Past Jedi. The problem with these points is as follows. With Poe the answer was, No, he isn't ready to lead the Rebellion as he nearly led it to its downfall for a Second Time, spent most of the film ignoring his duty as a leader of the Rebellion and the day was only saved when Lando showed up to save the day.
And as for Rey, the whole thing of her trying to connect with the Past Jedi is brought up at the start of the film and is then promptly ignored until the Very End of the Final Battle. And, What does this moment that the Saga has supposedly all been building up to amount to? Rey using Two Lightsabers to deflect Palpatine's Lightning back at him instead of just using one!
I guess you could count Lando finding and reconnecting with his daughter but the film never bothered to even start this arc and and you can't resolve something that was never even started. There's also Ben's "Redemption Arc" but it's so tacked on and rushed that all we get from it is him showing up to help Rey, which only makes things worse, and then him reviving Rey and then kissing her before dying, more on that in a second.
Everything else? We never learn what Finn wanted to tell Rey, the Republic is still gone and which means there is no central authority to help keep the peace. How Palpatine was brought back was never explained so what's to stop him from returning a second time and starting this $#!t all over again? And finally, Rey saying that she feels like no one understanding her and the film itself implying that only person she feels she has any real connection with is Kylo which is never resolved thanks to Ben's death.
At the end of Return of the Jedi, when Anakin turns on Palpatine to save Luke he tells Luke that he was right about him and dies as a Jedi. Thus we're given resolution to Luke's character arc, which was to redeem his father and become a Jedi himself, and to Anakin's arc which was to save someone he loved.
But ROS doesn't resolve anything. Sure the First Order is gone but again, what's to stop Palpatine from coming back and just picking up where he left off and now everyone's in a worst state then when the Trilogy started. The Original Trio are all dead, the Resistance new leader has proven his not capable of leading and the ending of the film, ignore all Supplementary Material, implies that Rey has left her friends and is now retiring on Tatooine and leaving her friends who, again, she doesn't feel she really connects to.
She-Ra ended with Adora embracing her friends and girlfriend as they prepared to go and restore magic to the universe. ROS ended with Rey back where she started, a loner in the desert who's more interested in the past then where she is or what she is doing.
And the reason this is a problem is because ROS was advertised and the end of the Skywalker Saga but it doesn't resolve anything anything it just stops. Again, it's not as bad as the ending of Mass Effect 3, but it's still pretty bad. As I've said in the past, a good ending is one that makes you want to go through the whole journey again, a bad ending makes you wonder what was the point of all of it. She-Ra, the First two Star Wars Trilogies and the Thrawn Trilogy are the former, TDST is the latter.
