Once Upon a Time doesn't pretend that King George's or Queen Regina's ruthlessness is a tragic necessity for governance in a world not made for saints. Or that their couldn't really be seven dwarfs because real people inevitably go off in different directions against all persuasion or fight each other because time passes. Or that supposedly good people are either irrelevant because they're all nerdy in the face of badassery. Or that their goodness is meaningless is they haven't got the balls to kill.
A lot of people have a worldview that is rather sociopathic when you spell it out, so they like to call it an esthetic preference for shades of grey or moral ambiguity, as opposed to childishness. It is definitely true that Once Upon a Time fails the dark and gritty test, no matter how dark the villainy, no matter how human the villains motivations and no matter how hard it is to be a hero. Because in the end the insistence on dark and gritty is all about validating the Politically Correct (in this society, reactionary) worldview.