It's also difficult on the reader, who must sort out what is important to the story from what is important to Benjy. A bad unreliable narrator is unbelievable because the reader can't understand how come the narrator hasn't figured out what the reader has. A good unreliable narrator is a fine piece of work. The narrator must be presented to the reader as someone who is working out of the context of their own personality, and who misses things that the reader can pick up because of the narrator's naivite, or prejudice, or prejudgement. Like Huck Finn, who doesn't realize that the two guys that he and Jim are traveling with are con men because he is a child and has never seen a con, but his description of them is sufficient for the reader to realize.
Every book has a narrator.
In first person books, the narrator is the 'I' of the book. But there's a narrator in third person books too. The more limited the p.o.v. But even in a third person limited p.o.v. where the story reports only what the characters sees and thinks and feels and experiences, someone is selecting those experiences, and the cumulative selections suggest a personality at work, a narrator. This gives the book a certain tone, a certain voice.