The problem with that sort of storytelling is that you have to spend more time on exposition. The fish out of water premise gives you an audience stand-in who needs everything explained to them.

"These eels are what everyone uses for currency. Their value is inversely proportional to how much they like you. That's why we use them to do battle, so they hate us. And if we win this eel whip race, the Tree of Sorrows grants a single wish! You can go home!"

If there's no Mary Sue, then you have to work those elements into plot points. It takes much longer, and you have to build slowly so you don't lose the viewers/readers. But if a regular kid is transported into the fantasy, then discovering everything is like solving a mystery.