What's stopping a nation for voting itself more than 0 UBI now?

What stops a nation voting itself lower taxes?

If every unscrupulous politician can promise to lower taxes to get elected, what stops that?

Politicians tend to promise to spend more taxes on projects that will benefit some segment of the population. In the UK "more money for the NHS" is a definite vote-winner.

Well, according to Benjamin Franklin (ostensibly),

“When the people find that they can vote themselves money that will herald the end of the republic.”

So basically, any nation that increasingly votes itself more money stops being a nation pretty soon

There's a more general problem. What's the right amount of UBI? Even with the best intentions someone will be unhappy.

True. But you can never please all the people all the time ;)

I guess the amount that will generate GDP growth..

In the end, it probably doesn't have to be perfect, just better than status quo.