I've purchased two bags from eBay (both international sales) that went through authentication. Both bags were authentic and as described, although IMO there were some water marks on one bag that weren't very clear in the listing. That bag was returnable and returning it was not an issue at all.
eBay insists that they use human authenticators that personally handle each bag. Someone else here on the forum had an authenticator reach out to mention that the bag being examined had some major undisclosed condition issues, so, this seems plausible.
If you are selling, know that eBay will un-package and re-package your item in their own bag and packaging (with a tag and a QR code that links to the authentication details); this happened to an eBay seller I know who is very proud of their own packaging. I don't know if you can ask them not to do this.
Since authentication is listed as "offered free for a short time" clearly either sellers or buyers are going to be asked to pay for the service at some point.