Far from being a win-win financial instrument, SIBs come with significant technical burdens and exemplify an ideological shift in welfare service provision. The ongoing hype around SIBs, their seemingly unstoppable proliferation worldwide, and the significant amounts of money investors are putting into them would suggest that they are widely regarded as the future of impact investing. However, we contend that such uncritical enthusiasm for the SIB model is a worrying trend. There are significant technical challenges to overcome in setting up and operating SIBs, which advocates insufficiently acknowledge. More substantively, however, SIBs fundamentally change the nature of public and social services, effectively reducing citizens to commodities.