UBI wouldn't eliminate all other welfare programs. Only the biggest and most distrusted ones.
We'd still need safety nets for people in non-standard situations which UBI can't cover. Healthcare costs are one that comes to mind. Without healthcare insurance coverage for people on only UBI, any hospitalization would break their bank.

Handling UBI for families with varying numbers of children is an issue that people will endlessly fight about.

Another thing UBI eliminates is minimum wage.