I don't know much about weather but something like herd immunity is definitely testable.

Firstly, even a smallish percentage of immunity would still see population wide benefits.

Secondly, as diseases are transmitted to people physically nearby, this could be tested by immunizing a large percentage of a single population, such as a small town full of people who don't travel much.

If you are truly concerned about the population wide effects of UBI, then you could do something similar, like fund a small poor town, and give EVERYONE in that small town UBI.

Unfortunately, I expect that of such a test were attempted, you'd see the same exact criticism of it, though. It'd probably be something like "you can't see the benefits unless the entire WORLD is on UBI!!!" or some other further doubling down on the unprovability of it all.

My point being, that this scepticism of actually testing the damn thing seems more like a defense mechanism against the possibility of being wrong. It is just the same lazy approach that everyone has for their pet theories, as a way of deflecting any and all criticism by declaring it unfalsefiable