The thrust of the (financial) problem is that all of those savings, the bureaucracy and the more efficient capital allocation, is completely eclipsed by the massive waste of paying UBI to everyone who doesn't need welfare. If 10% of households receive welfare, you're looking at a 10x increase in outlay (since you're now also paying the 90% who don't need welfare).

But to actually replace welfare, you'd need to pay enough to satisfy (almost) everyone, not just the average. This means you need to pay the 80th? 99th? percentile benefit to everyone, since UBI is a flat benefit and you've eliminated all means testing.

Assuming your 99th percentile outlay is 150% of the average, you now have to pay 1,500% of your original welfare budget in order to provide the same benefit to the 99th percentile welfare beneficiary (and 1% of people previously on welfare are still worse off). Even if somehow we could afford largess of that magnitude without spiraling into hyperinflation, it's unlikely that the efficiencies of individual capital allocation could overcome anywhere near the 1,400% premium of UBI.