All of the above is consistent with my experience as well (recently found a photo of a gravestone for a woman whose maiden name was Dewey, according to multiple official records - and on the gravestone it is spelled Duey, made me wonder whether the family had paid the stone carver in advance!) Also, a rather officious/difficult person created a Find A Grave memorial for my great great grandmother, which had her dying in Kentucky in 1826 (and buried in a specific cemetery, where her aunt and uncle and some other family members are buried). This resulted in my husband going looking for her on a visit to the area (in freezing weather), with no success. I then figured out that in fact, she died in 1900 in Solano County, California (and was already on Find A Grave, with a photo of the gravestone on which a daughter and grandson also are shown). Even so, it took weeks to get this guy to back off and remove the memorial.
Similarly, a many-times great grandfather was listed on Find A Grave as buried in a New Jersey cemetery (where a couple of his sons and other family members are buried), but he is well documented historically, and there is no evidence that he ever even went to NJ (he lived in York County, ME) much less died there. That person did not back off until several descendants (myself included) started putting smarmy notes/flowers on the memorial, saying things like "even though there is no record of where you are buried, it's nice that you are remembered," etc. The notes cannot be removed by the person who created the memorial, so eventually he was shamed into just deleting it. But as you can imagine, there are any number of Ancestry trees (and on other sites as well, I am sure), that now have wrong information thanks to that clown.
That said, however, the Find A Grave memorials can provide important clues to names, spellings, relationships, dates, etc. Once you have that information, frequently you can verify (or disprove) it through other sources, because you now know at least approximately what you are looking for. For instance, if the Find A Grave memorial includes descendants from more recent times, you may find that the descendants' official records include a mother's maiden name, etc.
I add Find A Grave memorial numbers in the descriptive section of the "death" event, which makes it easy to go back to them. Also, since I started doing this before Ancestry linked to Find A Grave, I have a manual citation form for Find A Grave links, into which I insert the URL for the particular memorial.
Good luck with it - you are wise to proceed with caution, no matter what source you are using. (And it is not just Find A Grave where there are errors - I have found some real bloopers in official records of various types.)