Interdependencies and inherent variability in Supply Chains must be addressed in its design.
Supply Chains are complex systems with inter-dependencies and variability. I have come across some designs which ignore these sub-system variabilities and work on standard lead times, standard processing times. Such designs will fail by design (pun intended).
When we are designing supply chains for a specific context, we must acknowledge and respect the market and internal realities, rather than ignoring them in the name of simplification.