I would disagree with that; you can formulate it. Using it is difficult except numerically; in my experience. Most any time varying function (including time varying coefficients) has a Laplace and Fourier transform. In the case of finite apature S/H or autozero systems you have to make up fancy equivalent circuits (time domain analogs of thevin equivalent circuits) and then write in transform equivalences. In the case of random noise or signals you have to resort to the power domain S(s)*S*(s) . Althought there is a systematic method for dealing with impulses of any order and position; I have never found a systematic way of creating the above equivalent circuits; for finite apatures and autozero circuit inclusion.
Ray