It seems to me that the question of how to finance a UBI is probably the largest stumbling block.
Of course it could be done through taxes but a lot of people have legitimate concerns about government taxation. For taxation to work you need a coercive government and many of us are very aware of the downsides of such an entity. Of course taxation isn't the only reason we have a coercive government there's also the war on drugs and other policies which seem to ultimately derive from the need of a significant fraction of the population to dictate the choices of their fellow citizens. This tendency is of course inevitably amplified among those who self-select for government or political careers. There is also to some degree a legitimate need for self-defense which also inevitably leads to some degree of coercive government. But even if these problems could be solved, say for example through more rational and better focused defense policies and through evolution of ethical views to give greater weight to the desires of individuals to lead their lives as they see fit, if we are going to provide a safety net for everyone we will always need some form of government and if that government is financed through taxation a level of coercive and invasive policies is likely inevitable.
Could we find a better way to finance government and a UBI ? Some countries and regions with extensive natural resources have been able to leverage them to help finance their governments and provide assistance to citizens (examples I believe include Norway and Alaska). However most economies are not so strongly dependent on the exploitation of natural resources so this type of solution cannot be generalized. What if a nation's government became an investor in that nation's economy ? I'm not talking about nationalization of industry but allowing the government to acquire minority ownership of public corporations as a means not of exerting control but of generating income. Could we imagine government evolving to the point of relying for its financing on a financial portfolio much like some universities receive substantial income from their endowments ? I'm not an economist but I've never encountered this idea in the press. It would be interesting to know what the experts think of it.