I'm not speaking ROI in the sense you seem to have taken it. Social welfare exists for compassionate as well as economic reasons. If we spend most of the money on the bureaucracy of the welfare system, then very small fraction of that money makes it to the intended demographic. This would in fact be a drain on the tax payer, with no return on the investment. 

UBI just sends everyone a cheque, and then allows the tax system to ensure the UBI benefits those it needs to.  This = investing in the consumer economy, rather than spending a bunch of money on civil servants, which I assume is your least favourite thing.