As the popularity of smart city research is increasing, the measurement of smartness became also a popular research topic. This is in accordance with the demand of the planners and project financing institutions for the success indicators, and with the observable tendency of new indicators for describing the settlements from the point of view of the quality of life, liveability, creativity, environment, social capital, development and many other fashionable research aspects of modern urban societies. Analyses through such indicators run the risk of taking a mechanistic, technocratic, superficial approach to complex urban systems, ignoring the complex causal relationships between urban subsystems and the interpretative and statistical uncertainties behind the indicators. Moreover, these composite or complex indicators merge very different basic indicators, often with low data quality and validity. This paper focuses on the uncertainties of smart city indicators, which are often used to form composite indicators that moreover form the basis for comparisons of smartness of cities. The transformation of a multi-indicator system into a one-dimensional metric scale is a highly questionable practice. Composite indicators, despite their popularity, are methodologically and conceptually highly problematic analytical tools for researchers and normative targets for policy makers.