Decisions related to system architecture are difficult, because of fuzziness and lack of information combined with often conflicting objectives. We organised an industrial workshop with the objective to choose 5 out of 800 architectures. The first step, the identification of selection criteria, proved to be the greatest challenge. As a result, designers selected system architectures that did not satisfy them without being able to explain what went wrong in their selection process. The objective of this study is to investigate the impact of criteria in system architecture selection. The recordings of the workshop were transcribed and analysed in order to identify the difficulties related to the definition and the use of criteria. The analysis highlights two issues: the interdisciplinarity of system architecture makes criteria interdependent and the lack of information is making it impossible to define an exhaustive set of criteria. This questions the applicability of most of design selection methods that assume that criteria are well defined by designers. Finally, this study provides insights and recommendations for future selection support tools dedicated to system architecture design.