Saudi Arabia does not offer UBI. They have a state-monopolized petroleum industry. Assuming it works as you say, this is still not proof UBI works. That political monopoly entrenches a political class, but at least there is actual industrial production of a market good, and not just money printing, behind such a scheme. It isn't sustainable, though. Even if the Saudis weren't deeply corrupt, there is still the economic calculation problem. We saw what happened when Venezuela relied on state oil monopoly. It was a miracle of socialist success, until it collapsed that is.