Suppose I was telling you a story. Suppose I said,
"Once upon a time, there lived a young girl with hair black as coal, skin white as snow, and lips red as apples. She lived with a wicked step-mother who happened to be queen of all the land, and one day the step-mother discovered that her step-daughter had surpassed her in beauty. So the step-mother sent her huntsman to rip out the young girl's heart. But instead the huntsman let her go, and the young girl ran away and stumbled onto a small dwelling where there lived seven vertically-challenged men."
And then I asked you to tell me what fairy tale this was.
Well, naturally, you'd respond, "Snow White and the Seven Dwarves," right?
And then suppose I went, "Hah! I lied. This isn't about a young girl with hair black as coal who finds herself keeping house for seven small men. No! It's actually about a young girl with hair bright as gold, who finds herself falling asleep in a house belonging to three bears. Fooled you!"
You'd be a little annoyed, wouldn't you? You might say, "That wasn't a fair guessing-game, you gave me all the wrong information! That doesn't count! You cheated."
Yeah, you'd be right. I totally cheated. And that's how I feel every time an author pulls a similar stunt with the narrative of their story.
This is one reason unreliable narrators are so difficult to pull off. Because to the reader, it feels like a deception, perhaps even a betrayal. Readers take what they're told at face value most of the time. I've heard it described kind of like an unwritten contract between the reader and the storyteller: you're going to tell me something fantastical, and I'm going to go along with it. You can't immerse yourself in the story if you're questioning the truth of every word on the page.
So when all of a sudden, the writer informs you that, Whoops! Actually that's not at all what happened, it can feel as though your trust in them has been violated. You think to yourself, "Well if they lied to me before, what's to say they won't do it again? How much of what they're telling me is the truth, and how much is just more lies?"
And really, that's no way to go about reading a story.
I find unreliable narrators particularly odious when they are used in mysteries. Because in mysteries, the whole point is for the reader to try to solve it, to piece together the puzzle before the characters do. And how can you possibly do that if the narrator is withholding information from you (or worse, giving you false information)? Say you're told that the protagonist is going to be interrogating a suspect and you know this will lead to a very important clue...and then the protagonist doesn't mention the outcome of this interrogation. All they'll say is, "I'd learned something that took my breath away. And now I knew where to place the blame." You'd be a little ticked off, wouldn't you?
The way I see it, this is just not playing fair. It's sort of a quick-and-dirty method of stringing the reader along so that when the big reveal happens, they'll be surprised.
Well, OF COURSE they'll be surprised. You didn't give them enough information to have them be anything other than surprised! (And annoyed, frustrated, and just about ready to throw in the towel, but I digress.) I'd say this is a very poor use of the unreliable narrator technique. It's a way to cover up sloppy mystery writing, and frankly, it's a bit of an insult to a reader's intelligence. Good mysteries are difficult to guess because the clues are so clever or well-hidden, not because you've flat-out lied to the reader or conveniently "forgotten" to mention something.
A distinction should certainly be drawn between playing on a reader's assumptions — which is perfectly acceptable, and indeed can be a very crafty way of fooling the reader — and outright lying to them (either by giving false information or by omission). And there are certain circumstances where unreliable narration can be used to good effect. It may be very important in revealing something about a character's personality or mental health (for instance, in All You Never Wanted by Adele Griffin or Holding On To Zoe by George Ella Lyon.)
But using it as a plot device to keep the reader turning the pages? In my opinion, it's a cheap trick.
Rely on your writing talent to create a diverting, suspenseful, twisty read. Don't rely on a technique that will only try the reader's patience. You might just find that they run out of it before they finish the book.
In which case, your "big reveal" will all be for nothing, anyway. Meanwhile, your reader will be reacting like this:
Tell me truthfully, now: is that really what you want?