A collection of second order effect questions that people don't really have a strong cohesive argument for or against:
1) What are the impacts of UBI on cost of labor, and how will businesses react or adapt? (will UBI ultimately lower cost of labor for businesses or will it raise it? Will it cause businesses to seek out investment into AI instead to replace manpower? etc)
2) Are there any situations where UBI's cost will spiral out of control? (Do we have smart enough people modeling out long term costs of UBI in a comprehensive, risk accounting way so it doesn't become another social security)
3) What is the change in percentage of people who will couch potato with UBI? How about percent change in entrepreneurial activity with UBI? (at the very least, is it in the single digits, double digits? A fraction of 1%?)
3rd order effects:
1) What will the economy look like 20 years from now with UBI? How will the structure of society change?
2) How will UBI change how the majority of people grow up and what they end up pursuing?
3) How will UBI impact the global economic climate? (My sci-fi dystopian vision of UBI's butterfly effects, not that globalization hasn't already done this: UBI will spur one country to develop so much AI due to lack of people in the workforce, and that AI is so good that it gets exported to all countries over the world, centralizing most of the money in the world to that one country, making that one country able to afford UBI while the rest of the world is jobless and has no money to take care of their own people.)
I'm just scratching the surface of the complexity of the topic.