I think a lot of people way overestimate the administrative costs. For example U.S. Social Security administrative costs are around 6 billion a year. Sounds like a lot but it's only 0.7% of the total cost of the program[1]. Sure there's a handful of different programs all with their own costs, but it's maybe 20-50 billion a year. That's not that much considering UBI might increase spending by trillions.
The lifting for social services is at the state and county level. There’s probably 3 million people engaged in that function, depending on what you scope into it. I betcha there are 10,000 people just doing children’s services in NYC.
Ok, but we aren't talking about UBI replacing CSA or really any of the local services are we? It's not like child abuse is going to disappear because everyone gets a UBI check. A lot of the state and county level social services are not going to be obsoleted because of UBI. I would think it's more of a federal/national program meant to replace federal/national social services.
Even something like CPS is significantly funded via other programs. They perform functions like managing foster care, early intervention, medicaid enrollment, etc and draw administrative funding that is shared among programs.
It doesn't mean the UBI is bad, just that you cannot bank all of the "legacy" spending.