* At the time when it aired, Serena was in need of screentime and focus. It begged the question as to why a gym leader, who's going to be permanently gone from the show after her gym episode, was getting her own arc, while Serena and even Clemont lacked one.
* Except for maybe Ash, Korrina's arc relegated the rest of the group in the background.
* Too much Lucario. While episode 32 was quite good, the arc shouldn't have been only
about Lucario. In the episode where the group gets separated, Clemont and Korrina were alone. Why not have a more human scene where they talk about being gym leaders and their own dream/goals, etc?
* Team Rocket appeared too much in the arc. Guess what episode didn't have them? Unsurprisingly episode 32, the best one. Why not create a unique arc where Korrina and Mega Lucario are able to bond through some actual antagonism? Like the gangster Malamar or Team Flare.
* Pikachu didn't need to battle that much in the arc. Fletchling and Froakie should have gotten the battles.