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.