Let us consider Pawn's Dream by Eric S. Nylund. Avonova, with a 1995 copyright date. (I see no earthly reason why those last two bits of info are important, but serious critics agree that those are the first two things any reader looks at, so I looked at 'em -- just now, about a week after finishing.)
In this book we meet Roland Pritchard, our first-person narrator. He lives with his crazy pyromanic mother, he's got the night shift at a 24-hour convenience store, and basically his life is on the border of nowhere. But he dreams -- and in his dreams he's a linguist in the Abbey of Glossimere, and has been for the last twenty years, studying ancient languages and lore. And that acolyte, when he dreams, dreams of a strange world of machines and cities. Get the picture? Yep, same guy, real dreams, living in both realms, the normal old US of A and fantasyland.
Anyway, here on earth he's being sought by members of a family, people with wealth, power, and magical abilities. And over in fantasyland he's planning to escape from the Abbey and seek out the marvels he's read of in the old scrolls. Great setup.
After that setup, though -- we don't go much of anywhere. And where we do go is this: in under a week of elapsed time, from a standing start, our narrator becomes the most powerful wizard in either realm, capable of doing things that no one else can do, and he singlehandedly ends the magical war that's been laying waste to fantasyland. All this without, in fantasyland, having actions or thoughts that you'd expect from someone who'd been a scholarly linguist for the last twenty years. Who, when he sees someone make a mighty leap, thinks of the Olympics, rather than whatever the fantasyland equivalent is. For that matter, in this world, this isn't a guy who has thoughts and actions that you'd expect of someone who's been working nights in a 7-11 for the last twenty years, either.
In other words, the fantasyland segments (every other chapter, as our hero goes to sleep in one realm and wakes in the other) aren't fully realized as a separate universe.
I'd thought that I'd go to great length about inconsistencies, about how our hero cures his monther's mental illness, only to have her burn herself down in her apartment (making me wonder about how good a cure it was); or his companion Smoking Bear in fantasyland, a lady who carries a sword, and who in the course of the book fights a demon, a stone ghost, another demon, and a brass man, without harming any of them. I'd have been more impressed if she managed to win one fight, somewhere, against something, so that carrying the sword wasn't just a waste of time.
I'd thought too that I might mention why I felt that this book was disgustingly immoral, and that the society of wizards, in fantasyland, all deserved to die and their realm revert to the demons, and that our hero had no right to make the final "choice" he made (the one for which all his comrades congratulated him, and sympathized with how hard the decision must have been). But I find I'm too tired to make the arguments.
Instead, all I'll say is that this book has deep structural flaws, which required rethinking the whole thing, and rewriting to a degree which was not done.