It’s an interesting debate about SF being written by mainstream writers, and whether it is still SF. Most of the early examples are female writers who coupled SF and feminism. Atwood, LeGuin, Lessing, Octavia Butler (who also brought in race topics, of course). Whether you see them as SF or mainstream really depends which editions of their books you pick up. And it really doesn't matter either way, they were (and are) just good. When it becomes embarrassing is when mainstream writers start playing with SF tropes but don't have the skill to carry it off well. At the moment I'm reminded of that point in the eighties when mainstream white pop acts started rapping - embarrassing to say the least. You can tell when an artist has a real grasp on the tradition they are working in. You don't expect classical musicians to be able to play rhythm and blues without at least listening to John Lee Hooker for a while, yet mainstream writers go stumbling into the depths of Hard SF territory without apparently reading any of what has come before. Fair enough if they can do it, but if Cormac McCarthy and Winterson are any guide it seems that they can't. What's "rebellious" about conforming to current expectations and ideology? Stereotypes and political correctness are two sides of the same coin, treating characters as statements or representatives and not as individuals.
Quite apart from believing there is space for pure entertainment, I also do not believe that interesting, challenging work usually comes about as a result of a writer sitting down and consciously thinking "OK, I'm going to tackle this important topic". Writing is more often a process of exploration and discovery, with a lot of unconscious input. As a provision, I would also suggest that the expectation that writers must "treat characters as statements or representatives and not as individuals", reliable narrators or not, is also a presumption and taste of our own particular time, place, and culture.
Why "must" this be so?
Are allegorical and symbolic modes of narration always somehow less rewarding? I’ve fed up with people saying they don’t bother reading books with unreliable narrators. Why? No idea. I think that the whole palette should be available to the writer and the reader. I also think that imperatives about making SF "representative" reveal the degree to which contemporary notions of Realism have saturated aesthetic discussions. Representative values and individuation are certainly not as necessary (or necessary at all) for the success of works such as Dunsany's ”The Gods of Pegana”, Cabell's ”Jurgen”, Eddison's ”The Worm Ouroboros”, or Lindsay's ”The Voyage to Arcturus”. And I would maintain that -- viewed retrospectively -- two works that I greatly admire, ”A Wizard of Earthsea” and ”Perdido Street Station”, now seem as much about "types of narration" as anything else. This is not meant to mark down Le Guin or Mieville. Far from it. Rather, I think that ”A Wizard of Earthsea” and ”Perdido Street Station” will endure despite their politics or ideology -- which will increasingly date over time -- by virtue of their style, tone, and aesthetic achievement.
When it comes to first person narrators aren't they all essentially unreliable? Who cares? David Copperfield, Holden Caulfield, anything by Gide, Mersault, and so on. Also, shouldn't we distinguish between 'twist in the tail' narratives where an objective truth is revealed in the denouement, and more nuanced novels that describe the fine line between knowledge, story telling, and madness.
I'll never understand the people, and I've met several, who say they don't like unreliable narrators. For me, they're the only interesting kind and K. J. Parker is, undeniably, the SF master of the form.
SF = Speculative Fiction.