It's simply: So far almost every experiment I've seen had positive outcomes in terms of employment, physical and mental health, overall happiness / stress reduction. That means we should keep expanding this until either we find actual problems to deal with or everyone is on UBI. We can spend ages trying to come up with various risks to rule out, but... what's the point in practice? We're basically agreed that we don't really 100% know how people will behave at scale. Worst case, we can discover and deal with real problems. Best case, it works great at scale as it does in small tests.