To cascade feedback loops, or not?

Linear time invariant (LTI) block diagrams—and the associated transfer functions—are profoundly inadequate for multi-loop control system design. A practitioner has to have a thorough understanding of the physical uncertainties—and of their inter-dependencies—in the different parts of an interconnected system, or in a given block diagram. Lack of attention to, or understanding of, such causal interactions has led to ill-advised reliance on cascaded control in practice—and to consternation when cascading fails to yield the expected improvement over a single-loop design.