If you can't break down your systems model into a subset of inputs, outputs, and domains (which can be broken down even further to whatever degree of complexity

If you can't break down your systems model into a subset of inputs, outputs, and domains (which can be broken down even further to whatever degree of complexity necessary), someone probably doesn't fully understand their system very well. Yeah, UML is chunky, but the ability to identify the high-level goals of a system allows you be be focused on how those systems should behave and the rest is just meeting that spec reliably (or within whatever margin of error is determined to be appropriate). If things are truly inter-dependent, the ability to identify risk points as a high, abstracted level is invaluable.