As an HN user, you would make more money with your current skill set in the current job market. With that in mind, your net value to society through said job would be higher than the value you would provide fixing the city with that UBI money. Before going to extreme UBI scenarios, it would be interesting to have private enterprises tackle projects that effect the public with public funds that are earmarked to go directly to actual individuals at those enterprises (versus retaining profit) or individuals in that locale.

I don't know, he could also work in ad tech and have a net negative impact on society. In that case just paying him to do nothing would be better. :)

That's mostly my concern, I tried finding IT related jobs for really useful things (anticrime, education) but failed. Most interviews I ended up going to were of basic recruiter kind and no social value IMO.