I recall that there was a similar experiment conducted during the 1970's in the Interlake region of Manitoba, Canada. The one obvious negative effect of this was a significant drop in labour participation rates among three groups; youths, women with children, and near-seniors.
The experiment wasn't even set up in such as a way as to expose the worst outcomes. Such a test would have to restrict the taxes needed to pay for the Guaranteed Basic Income to the same geographic area as the available benefits; allow benefits to those moving to that area; and run for two or three generations.
You've fallen prey to observer bias; that the existence of observers in one place and time means that observers must exist in most places and times. In other words, that the environs of any observer must represent a typical sampling of the universe, but observers will normally find themselves in environs that permit the existence of observers; which might not be a typical sampling of the universe at all.
I can illustrate this with an historical example; when it was determined in the 16th century that the stars were like our sun, and that the planets (wandering stars) were worlds like our Earth; many intellectuals adopted the view that all stars had planets and that all planets supported not just life, but sentient life. While it appears that most star systems may include planetary bodies; the second supposition is much less certain. Certainly there are no intelligent Martians, Venerians and Mercurians.
Experiments with falling objects, from the time of Galileo onwards, demonstrated that objects of differing weight, but the same shape and density, did indeed fall at about the same rate. Also objects of the same weight, but differing shapes and/or densities fell at different rates; thus strongly supporting the hypothesis that the differences in rates of falling were due solely to air resistance. The later experiments, conducted in vacuum, simply confirmed beyond reasonable doubt this hypothesis.