My response follows:
I hadn't touched the feminist issues (partially because IMPerception, people use "Mary Sue" just as much regarding male characters as female; this is probably a flawed perception), but I was remarking a few weeks ago that -except- for "self-insertion character" (the only legitimate "Mary Sue" hallmark, IMO), that the traits associated with a Mary Sue were exactly the traits associated with a protagonist in an adventure story.
That the slur is more often applied to female characters than male ones is, once you point it out, obvious, and very problematic as well.
I'm happy to use "Mary Sue" (or "Gary Stew") specifically for obvious authorial self-insertion characters in fanfiction who warp the story such that it's a fantasy, not a story. And only that.
It is lazy and deceptive to use Mary Sue (or equivalents) when referring to characters in non-fanfic; if the character is unbelievable or an obvious self-insertion fantasy, that should be criticizable without using the misappropriated shorthand.
The only exception I'll allow is when a writer has apparently written fanfic in their own universe (such as late Charles de Lint), in which case treating it as fanfic might make some sense [obviously, there's also commercially published fanfic; the Wizard of Karres, the Peter Pan sequels, various Oz things, etc].