A source should never be used without the genealogist asking:
How complete is it? How was it compiled? What could be missing?
FindAGrave is perhaps the perfect example. It only includes burials that have been submitted by someone who either took a picture of the tombstone or, in some cases, learned about a burial in the cemetery or another source. It’s a great resource, to be certain, and it is a great place to start, but by its very nature it can be incomplete. Not every burial had a tombstone, deaths before death records were not recorded, and not every death or burial gets mentioned in a newspaper, etc.
Every site or set of records should be used with the same concern over completeness–not just FindAGrave.
But you should always be asking yourself “how complete is this source?”