The increasing of products variety is moving companies in rethinking the configuration of their Assembly Systems (ASs). An AS is where the products are being configured in their different variants and its configuration needs to be chosen in order to handle such high variety. Two of the most common AS configurations are the straight-line and the U-shaped line. In this paper we want to first determine and define, through a narrative literature research, which are the factors that are relevant in order to compare these two AS configurations. Additionally, with the advent of Industry 4.0 (I4.0) technologies it is possible that these new technologies have an impact on these factors. For this reason, first, we investigated how I4.0 technologies impact these factors and then we saw how the choice of the AS configuration, between straight-t line and U-shaped line, change based on the impact of the technologies. The seven factors here defined open the opportunity for future research challenges.