I recall years ago a passenger ship grounding of the east coast of the U.S. Apparently a connection to the GPS antenna had broken and the vessel was DRing. Will an autonomous vessel be taking ranges and bearings or shooting stars to back up the GPS? Nothing is perfect and equipment does break and malfunction.
Also, what is to stop or, at least notify “authorities”, if the vessel is boarded? Sure there can be cameras and sensors but those won’t prevent a boarding. If some group targets valuable cargo well out so sea how long will it take to send the ship police to the ship? Even then if the ship is taken over who will know where to find it?