Nelly in Wuthering Heights by Emily Brontë is neither a completely reliable nor a completely unreliable narrator.
On the positive side, she had direct personal involvement, and was an eyewitness to several of the events she describes. On the other hand, she is not an omniscient narrator, and only has access to the events she experienced or the accounts other characters told her.
Next, she is emotionally involved with the subjects she is describing. Even though she does not stand to gain financially from one particular version of the truth (though an entertaining tale might evoke a better tip from Lockwood), she does have an emotional stake in making herself appear both knowledgeable and benevolent.
Finally, there is her character. She is definitely inquisitive, and has many local connections, enabling her to obtain a great deal of information. On the other hand, her understanding is limited, and as a servant, she is not privy to certain types of information exchanged among the adults of wealthy families, although she is an avid eavesdropper and gossip.