It’s not about sample quality or size - it’s about the behavior of a group when condition change, but the conditions didn’t change. For example, under a proper UBI I’d quit my job and start a business. Under this time limited experiment I never would have. In a proper UBI environment I might scale down my living to fit in only the UBI so I won’t be dependent on a job. Under a 2 year experiment I don’t.

There are some minor effects that don’t need universal+indefinite basic income to show up, but tbh those aren’t very interesting.

It would have been a more interesting experiment to fund just 100 people for life, with the same money, even though that sample would have been smaller! But even with say 10k people funded for life, you wouldn’t see the most important effects on the job market, prices, rents...