Since people born into wealth are "guaranteed money over their whole life", can't we study what they do with their life compared to what people who are not born into money do with their life? How or how not would this kind of study produce meaningful, legitimate results?
Another "problem" with this kind of study is that they are giving money only to unemployed people. Because it would be also interesting to see what people with jobs would do. I suspect a lot of people would switch from full-time to part-time jobs (at least that's what I would do).
There is some aspect of this kind of experiment/study to find out how individuals would perform if they didn't have the guaranteed income in the past but then suddenly do, but that's only meaningful for the transitional generation. If UBI were truly universal, it would need to be multi-generational. And how would those born with it, never having known anything else, treat/understand work? There's no such thing as "switching from a full-time to part-time jobs" in this context, or at least the impact and meaning of doing so is different than those who have a concept of what not having the guaranteed income means.