Multiplex is great...as long as everything is working....when some particular function goes bad,you may even find the repair very simple - just replace the affected unit or controller. The bad part is the price of that part.
I have seen things like a ground short causing the controller to go bad. When there is a fault, the controller cuts off that item for a time, then back on -- it see's it's still shorted and cuts it out again -- sort of like a computerized automatic resetting circuit breaker. This is supposed to protect the system, but I have seen it fry the controller when left un-corrected for too long. The controller is very expensive.