A representative sample features the same external validity issues you have in many (any) experiments, including in the hard sciences. That's part of science, and it would be unfair to attribute it otherwise. One experiment won't do it.

As for your other point, I guess the issue the lack of a hypothesis regarding basic income. You can't test "basic income" as such, and with such unspecific setup you are sure to conflate rather than illuminate mechanisms of relevance. In any case, it depends on your question whether you need to take income away from anyone. Are you going to start by examining distributive effects? Really? No physisict would test their airflow experiment on a flying rocket, while also trying to measure how he exhaust affects the wind in Costa Rica next week and saying 'something' about the future climate. They would start by putting their airflow gizmo into an isolated chamber - which is, afaik, more or less where UBI experiments are.