If a country with a population of around a million (or even as small as 100k) enacted UBI I would take those results to be representative of a societal change.
I honestly doubt you would. The typical arguments of:
it's not comparable to a country of 350M, they're barely as big as $cityWithOver1Million
their society is very different from ours
their implementation is different from what we could ever manage
the circumstances were different
would come around.
You're making exemplary conservative arguments to stalemate progress by creating a chicken and egg problem.
Won't accept results of change in a small environment because they aren't representative of change in large environment
Demand results of change in a large environment before applying them to large environment
Won't apply changes to large environment because results of change in large environment don't exist