MOSA (Modular Open System Approach) provides a framework for efficient and sustainable design of complex integrated systems (e.g., integrated modular avionics). In domain of embedded technology, the MOSA as-is does a good job in identifying modular software and hardware frameworks required to establish a common baseline for generic open architecture. On the other hand, it does not cover physical aircraft integration, integration methodology and other constituent elements essential for design of robust interfaces and integrated embedded systems, which are owned by OEMs and their suppliers. The definition of open interfaces is a key constituent in definition of MOSA-compliant architectures. An efficient system integration lifecycle requires unambiguous interfacing among hosted functions. Open interfaces and Ethernet are core system integration technologies and should be integrated and configured with other software/hardware framework elements, to enable hard real-time, real-time and soft-time application hosting. The system modularity and scalability required for incremental modernization and upgradeability over the lifecycle are at the stake, if the integrated embedded platform and design/configuration/ integration methodology do not prevent unintended cross-functional interferences. This paper outlines relevant system integration topics and challenges focusing on definition of key system interfaces, which can affect the system lifecycle and implementation of MOSA objectives. This text reflects on system integration decisions, which may limit governments, OEMs and Tier1s ability to fully accomplish or exceed MOSA objectives. The conclusion of this paper is a MOSA approach alone cannot completely rule out modularity challenges. An unambiguous definition of key system interfaces is viable only if the embedded platform design and system configuration tooling establish a solid technology baseline, such that the modifications do not require complete system retesting. This reality also suggests that the integrated platform provider / system integrator role cannot be easily interchangeable, due to the huge investments necessary to develop a robust integrated embedded platform and integration methodology, that can support unambiguous key system interfaces. The challenge related to key system interface robustness and system modularity may become an important research topic in the future - in terms of embedded platform design and sustainable organizational models tied to the integration methodology for large MOSA-compliant programs.