All UBI experiments that I'm aware of so far have been confined to a single country.

All the ones I recall are extremely limited on a number of fronts. They limited to a small group of people, for a limited time, and generally the funds are quite limited. I personally don't think these even qualify as UBI because some of the proposed positives for UBI do not exist when it has such limits. For example, if you are only getting UBI for 2 years, you have to plan out for the rest of your life after those 2 years and can't make the same choices as if you were being given UBI for life.

Fully agreed. UBI simply has not yet been tried on a scale and over a span of time that it would qualify as a real trial.