But again, as GP noted, isn't that a problem governments have already solved? If you want UBI it's going to be through the government, as that's the entity collecting the funds through taxes that would be distributed, and they already have a unique identifier for people. In the U.S. it's your SSN, which is conveniently also how they track you for income taxes (again, where a large portion of the funds for distribution are gathered)
If the government wants to link a retinal scan to a SSN, that might make sense with regard to UBI, but how does it make any sense for some random blockchain? Are we expected to think that even if a government wanted to use a blockchain they would use some third party blockchain and not roll their own? I find that highly unlikely.