The SZ effect has the benefit of being redshift independent, since the cluster gas produces a fractional change in the CMB that depends only on its Thomson optical depth, i.e. on its temperature-averaged electron column density - in other words, on the cluster pressure integrated along the line of sight. Integrated over the cluster, the SZE flux measures the temperature-weighted mass of the cluster, its total thermal energy. The X-ray emission on the other hand depends on the integrated emission measure, and it is a complex function of temperature.
The SZ and X-ray brightness scale very different with redshift, as former scales with the angular diamter distance, latter with the luminosity distance. Between redshifts 0.5 and 1.5 this makes a difference by a factor 8.