They are equivalent under certain assumptions, but there are still differences (assuming negative income tax would operationally work in the same manner as current tax systems), e.g.:
1) UBI is targeted to citizens, while income tax affect tax residents, these may be different groups of people.
2) tax refund is paid once a year, which would be problematic for people with bad money management abilities.
Ok, then let's go with a negative income tax instead. Other than avoiding welfare traps there is not much merit to the idea of a UBI anyway. People have always worked, the only ones that really need a UBI are those that cannot do work or are in the process of seeking better work but are trapped in their existing one. A negative income tax serves them just as well but people won't associate it with "free money for everyone".
I know a friend who would love to work as a software developer but can't afford even the relatively small tuition that a German university charges its students with his current job. A negative income tax would help him out.
For example, a UBI of $1000 and 25% tax is the same as a (possibly negative) income tax of 25% above $4000. That's because:
You can convert from one to the other.