They end up homeless and begging or stealing to eat.

So, it sounds like UBI would be effective in reducing overhead, but not much else will change if people choose to blow their UBI?

UBI does more than reduce overhead - it gives better incentives and options for people who can't commit to a substantial number of hours per week (at least not initially) to be able to contribute to the economy. But it doesn't solve every problem and shouldn't be expected to.