  Well--defined criteria are proposed for assessing the accuracy of quantum
master equations whose memory functions are approximated by Pad\'e resummation
of the first two moments in the electronic coupling. These criteria partition
the parameter space into distinct levels of expected accuracy, ranging from
quantitatively accurate regimes to regions of parameter space where the
approach is not expected to be applicable. Extensive comparison of
Pad\'e--resummed master equations with numerically exact results in the context
of the spin--boson model demonstrate that the proposed criteria correctly
demarcate the regions of parameter space where the Pad\'e approximation is
reliable. The applicability analysis we present is not confined to the
specifics of the Hamiltonian under consideration and should provide guidelines
for other classes of resummation techniques.
