I think many proponents of UBI overestimate the ambitiousness of the average person.
Nobody in my family has ever taken even minor risks like say, traveling abroad (for work or fun), starting a small business etc. I'm the first and I am only taking baby steps now.
Point being, we can't expect to give UBI for a few months and suddenly expect people to take big risks or become ambitious or attack huge problems. These things take time. Imagine a person who has seen his parents, aunts, uncles, grandparents etc being only salaried workers all their life, rarely moving out of their comfort zone. It wouldn't be easy for such a person to suddenly change (climb Everest, start a biz, spend a year writing a novel etc), even if their food and lodging is taken care of.
I don't know whether UBI is a good thing or bad. But I think both the for camp and against camp are too quick to jump to conclusions - these are social experiments not Math problems. They will take time and we can only learn by trial and error. And what works in one community might not work in another, even within the same state/country.
And I suspect that it will like result in even more ressentment from people who chose to stay at home on a meagre UBI against people who chose to work and earn a lot more. In other words it could have a politically explosive outcome (populist, socialist parties getting elected, etc).
I think many proponents of UBI overestimate the ambitiousness of the average person.