One of the issues is that UBI is different things to different people. Many argue that it will replace almost all other programs, but only if you want the high school dropout that'll take the "one bedroom studio, ramen, and a bag of pot and an World of Warcraft subscription" for a semester or two and lump him in with the faultless traffic accident victim who's fucked up pretty bad and has no realistic prospect of being employed ever again and will have to live at that level of poverty and zero additional aid for the rest of their lives.
Or we could start making value judgements about who "deserves" our help, who are capable of contributing, who has contributed, who's looking to contribute and what help have they already gotten and then we're pretty much back to the complexity where we started. I already pay taxes. If you want to give me a "UBI" all you have to do is adjust the income tax curve, paying out a UBI and then taxing me more to pay for it is just an exercise in how to create black labor and tax fraud.