Since the target audience was rhe unemployed, changing up their type of income isn't going to give them a job, so prpductivity is difficult to measure.
The entire point of UBI is to secure everyones basic human needs, not get them a job (unlike unemployment benefits).
If we were to test UBI also on employed people, and observed their behavior, we might avtually gain useful insight into mechanica with UBI. Perhaps we will see more 6-hour workdays with more people employed?