Doctor When wrote:
It’s not that the sequels are “undercooked”, it’s that they are raw. For example, it takes Luke 2 films to confront Vader, and even then he is soundly defeated. That Luke goes from swatting Wampa’s to duelling Vader in the space of a ‘few days’ is just the economy of filmmaking. The film (TESB) is at pains to show Luke going through intensive training with Yoda. That development is a significant and specific element of the plot and film. Rey on the other hand, goes head to head, and soundly defeats, Kylo Ren without any training... indeed, she’s never even used a lightsabre (she doesn’t even have any real lessons from Luke in TLJ). So it’s really not comparable... the sequels aren’t being economic, they simple avoid building the narrative. The net result is, IMO, that characters and situations are undermined by poor writing. They are not believable and the drama, which these films rely on, lacks credibility. I’d agree that TFA does a good job of establishing the main characters, but they are just not developed... either within TFA or TLJ.
You're making an argument that ESB's internal logic adequately sets up that Luke should be able to take on Vader. That his time on Dagobah, however brief it is, was enough for the audience to buy that he has grown as a warrior. Obviously, I agree because I too understand that in a 2-hour movie you can only do so much, and that screen time should be used primarily to deliver drama, not exposition.
Early in TFA, Rey is set up as a scrappy fighter and a survivor. Someone who is clearly capable of handling herself with a weapon. Is this the same as being proficient with a lightsaber? Arguably not, but we'll get to that. Later in the film, when she takes on Kylo Ren, a few important developments have occurred:
1. Kylo Ren has been mortally wounded after being shot in the stomach by Chewbacca's bowcaster, which the film sets up on multiple occasions to have explosive power
2. Kylo Ren has been slashed in the arm with a lightsaber by Finn
Despite this, and the fact that Kylo is basically bleeding out during the fight, Rey is never able to control the action: she's being overpowered, toyed with, and is retreating for all of the fight before the film decides to give her a "hero moment." And although she's been set up as being fairly proficient with a staff as a weapon, she awkwardly handles the lightsaber during the fight. So, to compare Rey "soundly defeating" Kylo Ren to Vader "soundly defeating" Luke is disingenuous.
If you want to argue that she shouldn't have been able to use the Force to grab the lightsaber, or perform mind tricks and all that, I'm there with you. JJ has said that she actually starts to learn how to use her power during Kylo Ren's interrogation where she taps into his mind, but I don't think that's properly developed. If it were, I don't think you'd have the same level of speculation that she had to have either been from some special bloodline or that she had been previously trained.
I would argue, however, that Rey was more developed in TLJ from a Force power perspective. Luke did teach her how to connect with her power more deeply and acknowledges that she truly has rare strength. There's also an entire lightsaber practice sequence. Again, using your ESB argument, I can draw just as much from that as I can Yoda's two training sequences. Not to mention Rey still looks sloppy in her fight with Snoke's guards, and doesn't dispatch them so easily.