An interesting question: how would one measure traditional " frequency response" to get Gain-Margin & Phase-Margin?
If you add too many extra components to create a Test circuit, then FRA will be their response perhaps not of the switching circuit.
In fact, does FRA apply to such nonlinear feedback systems?
What other methods do we have to demonstrate the "stability " of these circuits?
Can we set up to measure "output impedance" & "input impedance" ?