i wouldn’t call either of those unreliable as such. Nelly is biased, and her eyewitness account of the events she witnesses is very obviously partial, often unsympathetic, because of her loyalties and dislikes, and the fact that she’s a servant supposedly loyal to her various employers, and because she’s trying to minimise her own involvement or culpability in events she presents herself as retelling objectively, but nothing she recounts as factual is actually contradicted by one of the other narrators.
Like the fact that she ends the novel saying she can’t imagine how anyone could imagine anything other than quiet slumbers for the sleepers in that quiet earth, which is obviously what she wants to believe. But it’s juxtaposed by the account of the child who doesn’t want to pass the churchyard because of the ghosts.
The unnamed narrator in Rebecca is interesting. You can read her as a narrator who speaks honestly, truthfully passing on her account of events, which is only made ‘unreliable’ by the fact she’s lacking key information and has constructed a version of Rebecca that is reliant on her subjective interpretation of the Rebecca/Max marriage which she constructs from clues she misinterprets. In this reading, once she has all the information, she’s no longer ‘unreliable’.
On the other hand, you could read her as only becoming more unreliable once she has Max’s self-justifying ‘confession’, where she accepts his account on no other evidence than that she wants to, and from the moment of her well-timed faint at the inquest, actively aids and abets the cover-up of a crime.