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