That when we go into a book, we want to trust the narrator to guide us through this new/strange world. However, I think that here and there an unreliable narrator, or even a misinformed one, is useful too, and can be fun.

Up to the point of Lord of Chaos fans of the WHeel of Time will remember that Stilling/Gentling could not be Healed. It was one of the truths of the world. Nynaeve Healing Logain is a memorable scene for me because we had the rug pulled out from under us, even with the heavy forshadowing leading up to it.

So yes, I think it is a good thing if the unreliable narator is done well, and not just used as a deus ex machina, by the author for potential mishaps.