The standard counterargument is that the current welfare system uses a shambling bureaucratic apparatus which forces people to go through an onerous, dehumanizing process of filling out endless forms to determine whether they're the "deserving poor" or the "undeserving poor". UBI, in dispensing with with this expensive process, is claiming back money that's essentially just being hurled into a void for no reason.

I have no idea whether that's enough to make up the difference.