How worthwhile are certificates of authenticity in establishing provenance and value?
Like all documents, it depends on the value of the signature.
If the certificate is printed, including the signature, it is just a guide.
For things not worth forging, like Franklin Mint collectible medallions, it is all anyone needs.
For an oil painting that has a paper with a signed document on the back, faking the signature would be forgery. Still could be forged, especially if the signer is now deceased or impossible to locate by name alone.
But if the signature only states that the seller is dealing in good faith and not personally able to testify, then that’s all you can count on. Perhaps the dealer was duped.