Your FJG criticisms seem to indicate such a program would still be worth it despite the obstacles. Any FJG proponent will readily admit that such a large program will require an equally large administrative effort. These additional ‘costs’ are only costs in the sense that they require labor. But that additional labor is also employment, and it’s employment for the public good.
The criticism that the FJG will attract some workers outside of its ‘target’ demographic, and therefore be less effective as an anti-poverty program, is infinitely worse under UBI as the UBI benefit is universally distributed in equal measure.
I think the criticism about the FJG impact on the non-profit sector is a bit weak as well. Sure, some non-profit workers will move to the FJG and non-profits will have to compete with the new effective minimum wage. But, if implemented well, the FJG will deal with the problems many non-profits already target. And it will do so without the massive value-extraction inherent in the non-profit industry.