Even if every country offered UBI simultaneously, which isn't possible, they would offer very different amounts. UBI is welfare and acts as an immigration pull

Even if every country offered UBI simultaneously, which isn't possible, they would offer very different amounts. UBI is welfare and acts as an immigration pull - it cannot be actually universal despite its name. There must always be many conditions on who receives it, hence, it still requires extensive means testing. But in practice some countries will go first. And they will become far more attractive to immigrants as a consequence, and will need to have a plan for that.