Where I live, if the introduction of a UBI was followed by a drastic reduction (or complete elimination) of the means-tested system we know today, hunger would potentially become a problem for people blowing their money early in the month (or getting robbed?).
This tells me the utopian ideas of replacing the entire complex system with a simple UBI isn't feasible. You'll end up in dreadful ethical choices and moral hazard. This is indeed already happening, but the bureaucracy of it makes abuse less attractive.