I'm hesitating to get into it because of recent threads.
At least in my post, Arthurian folklore and its random treatment in various retellings, have created an incoherent mash of expectations and setting elements -- and this is referenced as an example of how story or system or campaign without an underlying foundation of internally coherent setting and expectations results in a movie or book that the audience is thrown off by, or a campaign that creates repeated "what, what?" reactions for the players.
From my POV, if the nature of the setting, or the abilities of the characters, are wildly variable depending on the "needs of the moment" in the story or campaign, so that we the audience or we the players have no idea what to expect, and our differing expectations are all both met and violated in turn from moment to moment, then that results in a mess. Further, if the system and fiction layers don't produce compatible expectations and results, you get the same problem.
Start with what sort of world you're running in, and what sort of things characters are actually capable of... balance exists as much at the fiction level as it does on the system level.