In the case of low material loss, the Nicolson–Ross–Weir method is known to be unstable for sample thicknesses at integer multiples of one half wavelength due to resonance phenomenon. Improvements over the standard algorithm have been presented in engineering literature to alleviate this effect. Furthermore, complete filling of a waveguide with sample material may pose a particular challenge: presence of gaps during the filling of the waveguide section would excite higher-order modes, which may yield errors in scattering parameter results. In such cases, more advanced methods based on the rigorous modal analysis of partially-filled waveguides or optimization methods can be used. A modification of the method for single-port measurements was also reported.
In addition to homogenous materials, the extension of the method was developed to obtain constitutive parameters of isotropic and bianisotropic metamaterials.