Sensorless drives for permanent magnet and reluctance synchronous motors are an interesting option in modern drives. Modified observers, such as the ones based on extended electromotive force or active flux, are used to get a unified model of the motor. The fundamental frequency based observer is of easy implementation, although the reliability of the model is undermined by parameter variations and corrupted measurements, especially when anisotropic motors are adopted. This study aims at analysing the influence of all motor model uncertainties, by means of an analytical sensitivity analysis. The resulting functions and graphic plots are useful to draw interesting considerations about the sensorless drive, in terms of motor selection and control design.