Rey already had the training she needed watching Luke fail according to Last Jedi, but all of a sudden we need Leia to train her. Leia suddenly a Jedi Master who never was even a Jedi Knight.

Having Leia train Rey was one of the most reasonable decisions JJ Abrams made, given what he had to work with. Rey watching Luke be a failure doesn’t count as training, or prepare her to be the grandmaster of a new Jedi Order. Her being trained by Leia, as out of left field as it is, is at least an attempt to make Rey’s power and Jedi status plausible.