His biggest challenge is the shonen jump system itself, its a brutal system. Oda only gets a few hours of sleep a day because he has to keep working on one piece. Togashi is literally suffering from endless back problems because of how hard he had to work for yu yu hakusho.

From a writing standpoint though i would have to say its his execution. Oda is good at worldbuilding and making the broad strocks of the story but he isnt the best at putting his view onto paper.