During an online exercise about searching online, I discovered that the biggest cemetery in the Phoenix Metropolitan area, the Greenwood/Memory Lawn Mortuary and Cemetery is vastly underrepresented on either FindAGrave.com or BillionGraves.com. According to these websites, there are just over 24,000 interments. But there could be over 100,000 burials. The Arizona Gravestone Project shows 2493 burials.
I began to wonder if this lack of coverage extended to other very large cemeteries? In checking for cemeteries on FindAGrave.com, I discovered that other large cemeteries were in a similar condition. The number of graves photographed of the total was very limited. It is probably a lot more satisfying to do a smaller cemetery, you get "finished" more rapidly and you can see some progress. Huge cemeteries such as the one in Phoenix are the special case. I guess the challenge is to organize projects to photograph the rest of the big cemeteries.
But another important fact from this information is that a failure to find a burial on one of these large online sources is not conclusive that the burial has not occurred. Another issue is the fact that the online programs such as FindAGrave.com focus primarily on the gravemarkers and not on the total number of burials. If the graves are unmarked, they are much less likely to show up in the online programs. Most of the larger cemeteries I have visited have a pretty comprehensive list of the burials. But in many instances, at least for private cemeteries, there are no complete records outside of the cemetery office. Unfortunately, you can never be sure that the online databases entered by users are exhaustively complete. Even if they are complete, there is no sure way to tell that fact.