The demand of metrological traceability to the International System of units (SI) for Micro-Electro-Mechanical Systems (MEMS) microphones poses new challenges for the development of reliable, fast, and cost-effective calibration methods. This work proposes novel solutions to meet these requirements, improving the capabilities of secondary calibration methods by pressure-comparison. Specifically, it addresses the effects of acoustic field irregularities and different frequency response of microphones under comparison in the determination of MEMS microphone sensitivity in the reverberation chamber. We present numerical techniques for predicting the sound pressure distribution within the narrow gap between reference and test microphones, along with the experimental evaluation of the deviation of calibration results when microphones of different frequency response characteristics are compared. The validity of the measurements in the reverberation chamber, is demonstrated comparing the calibration results obtained by different implementations of the pressure-comparison method, like in closed pressure-couplers and in the anechoic chamber.