Global university rankings are critically discussed concerning their outcome, e.g., pointing to contradictory results. In contrast, this paper examines three issues with the example of three well known rankings (Shanghai, Times and Taiwan). First, on which methods, in particular on which indicators are they based? Second, are there fundamental deficits of concept and systematic biases? Third, can one predict good or bad ranking scores, based on the respective concept? In addition with the example of German universities there are two factors on performance measurement discussed, disciplinary cultures and size of the university. In sum, the usefulness of global rankings is considered to be highly questionable. Measures in science policy should not be legitimated with such rankings.