It fails because if one wants to reject a quantitative approach toward assessing prose quality, then one would have to also reject out of hand the notion of "good story" as being something that is defined by itself. Since the former (the prose)'s rejection would constitute a further rejection of anything other than a relativist approach, that in turn would lead to exceptions being made in order to try and slip out of the counter that if there is no real quantitative approach and if it is all relative, then there could be no cause to use pejoratives to denounce those who utilize their own judgment to assess the poorness of the work at hand.

Now assuming that you really meant that there was this ill-defined notion of "good story" as being like obscenity, which is "I know it when I see it," then the question arises of whether or not others may reject your evaluative tools as being insufficient. Here, like in civil law, judgment would weigh on "preponderance of evidence" and not solely on individual perceptions. Now I suppose I could quote examples from what I have read in the Legends trilogy in support of my assertion that the prose is subpar in comparison to other fictions originally intended for an age 10-16 audience and perhaps you could counter that with....with what?

Here's the crux. I'm willing to provide evidence from the text itself. Are you going to do more than just say "good story" without qualifying it?

Then there's another angle, one that I think is the easiest for all involved. For this approach, one can simply admit that based on communal "standards" (which have some leeway, but are not so flexible as to admit all things), there are writings that would be considered poor based on societal expectations of what constitutes "good writing," but yet due to other, perhaps more idiosyncratic individual values (primary of plot over theme/characterization/prose, particular theme at the expense of prose/characterization, etc.) there are elements that one can enjoy while admitting that for the majority, such a work would be "trashy" or "poor." That is what I would do when asked to explain why I enjoy watching professional wrestling