Specifically, this experiment eliminates (much of) the problem that earning a few hundred euros will cut your unemployment benefits by the same amount. It allows people to take part-time jobs without being penalized. It's not perfect for the reasons you give and more, but it might be significantly better than what is currently in place.
The premise that one can find some universal income amount X and do away with all other types of benefits is a pipe dream anyway. For some, X will be enough to scrape by. For others, X will not be enough for their medical care necessary to survive for two weeks. There will always be a need for some extra needs-based benefits.