As to being impossible to model, there’s a great number of countries in the world eventually one will likely try the experiment on its entire economy without impacting 99.9% of the global population.

That said, you can turn individual safety nets into UBI lockstep with changing the tax code and see what happens. The fear is people would spend ‘food’ money on drugs, or people in subsidized housing would move, or all the things which created these programs instead of handing out cash. That’s a political question not an economic one, are we trying to act as a parent or do we want people to have freedom to make their own choices.