I've only read a little bit of M. John Harrison, so I may be off the mark, but I wonder if the divisiveness could be because it's marketed as SF, but it doesn't fit into the genre, similarly to Michel Faber, Gene Wolfe, or Vladimir Sorokin.
But I get the impression that the genres that sell well overall does so in large part because of binge readers that reads broadly. A client of mine runs a small press - I help them with tech stuff; small enough that they can "see" in the stats how often people buy an entire 20+ volume set of everything a given author has published at the same time. That doesn't happen in literary fiction, not least because few writers in literary fiction write enough significant books for it to happen.
Overall I don't really subscribe to this binary between literary and genre fiction -- I think there's a fair bit of speculative fiction that is serious character-driven art, such as the works of Ursula Le Guin. I'd also recommend Cloud Atlas by David Mitchell (made into a movie by the Wachowski sisters) as a lovely work of form-bending fiction that's hard to classify as one to the exclusion of the other.
In literary fiction, the order of importance is reversed. Prose quality is paramount, closely followed by characterization, with setting existing only to serve the characters, and plot hardly relevant at all.
This on the surface makes for boring works, but prose quality actually has the highest skill ceiling of all, both for writers and readers. "Quality" in this sense means "skillfully using words in the manner of literary fiction", which is difficult to detect if you don't read it, but it's a real thing. As appealing as it to assume it's all a big scam, the best literary fiction writers really are better, and the readers can tell, just like you can detect AI slop at the other end of the scale. I imagine if you put enough effort in it's even possible to enjoy reading Finnegans Wake (the final boss of literary fiction).
Plot/setting/character have real life as a reference, making them easier to master.
Do you mean some people find litfic boring? Otherwise, I’m not sure what you mean. E.g. there’s plenty of litfic with exciting, fascinating, and/or gripping plots. M. John Harrison, Doris Lessing, E.L. Doctorow are a few of my favorites in that vein.
Of course someone who finds the nature of the prose itself boring might consider the works as a whole boring. But that’s like saying someone who only sees in black and white finds artworks to be monochrome - it’s a tautology.