The calculation of the SERS enhancement factors (SERS-EF) is one of the most important parameters that characterize the ability of a given substrate to enhance the Raman signal for SERS applications. The comparison of SERS intensities and SERS-EF values across different substrates is a common practice to unravel the performance of a given substrate. In this study it is shown that such a comparison may lack of significance if we compare substrates with very distinct nature and optical properties. It is specifically shown that it cannot be compared SERS-EF values for static and dynamic substrates, and that the optical properties for the last one show strong dependence on the metal-molecule interaction dynamics. The most representative experimental results concerning the dynamic substrate have been supported by Generalized Mie Theory simulations, as a tool to describe the substrate complexity, microscopic information that is not usually taken into account.