Despite the attention sustainability-related urban measurement and assessment methods have received it is still not well understood how accurate (or not) the various methods are; their limitations in holistic city performance assessment; or, how they can be effectively used to better the design of the urban environment, city services and policies. Necessarily, urban measurement and assessment methods focus upon what is known. However, reflecting upon the unknowns and their impacts has the potential to deliver crucial insights into the assessment of city performance and governance. To this end, this study applies and critiques the city performance measurement and assessment method UK City LIFE1 in order to explore the challenges of, and prospects for, filling these gaps. UK City LIFE1 is designed to measure ‘livable sustainability’ at the city scale for the purpose of aiding UK policy makers and urban design decisionmakers. Results suggest that definitional uncertainties, the availability and viability of data, and the design of the method introduce inaccuracy, uncertainty and bias into data interpretation. This, combined with the complexity of city systems and the nascent ‘science of cities’, prevents causal effects from being fully described, potentially rendering decision-makers impotent. However, the language of ‘realizing the multiple benefits of interventions’ and ‘coupling and uncoupling relationships’ alongside making the unknown explicit has the potential to empower decisionmakers in the face of absent and disconnected data and interpretational challenges.