When readers talk about unreliable narrators, they usually mean one thing: perception. The narrator misjudges, misunderstands, or misinterprets events. His senses are flawed. His impressions are distorted. Reality leaks through at odd angles.
One of the quiet achievements of The Fall of the House of Usher is how long it refuses to become the story readers expect. Modern summaries rush toward the premature burial, the living corpse, the collapsing house. Poe does not. He delays. And that delay is not ornamental. It is ethical.