1) Does not solve the issue of 'meaning' and 'purpose'. That is, now that UBI (and automation) has gotten rid of the need to work for a living, what is left? Said another way, why not fall into alcoholism, drub-abuse, and crime? We don't have any good examples of large populations living off the government dole and thriving.
2) UBI isn't actually that much of an improvement over a well-funded welfare state, which leads to #3 ...
3) Does nothing to help developing world. First-world countries don't actually need UBI. They already have deeply funded welfare states. How does UBI help a laborer from a developing world compete when his labor is no longer required? How does UBI help a government of a developing nation build prosperity?
3) UBI is ill-defined. There is tepid acceptance of it on the left, but only with the implicit assumption that it doesn't replace most (or any) current welfare programs. UBI's support on the right almost always comes with the assumption that it will replace much of the welfare state.