Are you saying or implying that all switching regulators are unstable? Switching regulators are inherently non-linear systems and our notions of what constitutes stability are different from linear systems. In a linear system the root locus technique showing that the roots of the characteristic equation are confined to the left half plane is sufficient to demonstrate stability. In a non-linear system there is no useable transfer function nor characteristic equation.
Our notion of stability, for a non-linear system, centers around the outputs being bounded for any initial condition and forcing function. This notion is easy to articulate and somewhat more difficult to apply.