Why can't UBI be attempted on a local scale first? We have state and local governments for a reason. Something as risky and expensive as UBI, if it is going to be implemented, should be implemented gradually with proper testing, feedback, and adaptation.
Most countries which are not the Unites States don't have the power to make large scale systematic changes that this requires. It might not be possible even in the US, what with states shipping homeless people to other states and all.