To enable the production of high product variety, mix flexibility in assembly systems is of paramount importance for manufacturing companies. Mixed-product assembly lines (MPALs) are growing as the key means of realizing mix flexibility in many manufacturing sectors, as they absorb volume fluctuations and offer high product variety. With the increasing product variety in MPALs, these assembly systems are becoming more complex. However, the practical challenges of these assembly systems, in particular those concerning product design, have not been adequately addressed. By performing a case study of a heavy machinery manufacturing company, this paper investigates the implications of realizing mix flexibility in an assembly system for product modularity. The findings pinpoint the low level of product modularity in assembly as the most important challenge in MPALs. Accordingly, realizing mix flexibility in an MPAL impacts product modularity through establishing a common assembly sequence and defining similar module contents across distinct product families.