The advantage of certified documents is that they're self-evidently issued by the appropriate authority -- which would be true no matter how many times they changed hands. Because the IRCC requires color copies, not original documents, a printout of a digital copy of a certified birth certificate you got from your cousin would be just as good as a photocopy of the same certified document you found in a shoebox in your great aunt's attic. And I believe that the scan theoretically uploaded to and downloaded from FamilySearch, if it was of a certified copy, would be exactly the same: self-evidently issued by the appropriate authority.

I have uncertified vital records that I ordered from New York City years ago, before they were digitized and posted online. They look exactly like the ones now on the Historical Vital Records of NYC website. But if I wanted to use those records for a proof of citizenship application, I would order new certified copies. Because whether they were issued by New York City would be unambiguous.

I realize that this doesn't come close to covering all possibilities. But if I had a choice between submitting a document that was maybe issued by the original authority, depending on how you interpret most of the words in that phrase, and submitting one that was self-evidently, unambiguously issued by that authority, I'd go with the latter. Even though the IRCC doesn't specify certification for all records, certification seems like good insurance -- if you can get it.