Montresor is not very reliable. He is blunt to the extent that he explains his thirst for revenge and then goes on to explain exactly what occurred when he fulfilled that thirst. However, he is unreliable because he never reveals to the reader what Fortunato has done to him. He only says that Fortunato has, among thousands of previous injuries, insulted him. And Montresor adds that his revenge will not be complete unless he can "punish with impunity." This desire for revenge is the only impulse/motivation that we get. Montresor provides no information about how Fortunato has wronged him and he provides no information about how he feels following the murder and the subsequent fifty years since.
The narrator, Montresor, is reliable to the extent that he explains what happened during the deception and killing of Fortunato, but Montresor is wholly unreliable to the extent that he does not include Fortunato's sin, whether he deserved such a fate, and whether Montresor ever had any guilt about the act. The only suggestion of remorse occurs in the final paragraph:
I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick--on account of the dampness of the catacombs.
The -- suggests a hesitation: Was Montresor about to express some guilt but then lied, blaming his "sickness" on a physical reaction to the dampness? Montresor's unreliability is what makes the story so intriguing. He/Poe leaves the reader wondering about what led up to the revenge and about Montresor's reaction following it.