The value of capacitance drops only slightly as voltage is applied to the varistor. As the voltage approaches the nominal varistor voltage, C drops abruptly and R approaches a fixed ESR value.
Wheras diodes, i believe the C²V product is a constant figure of merit as well as tolerance for reverse bias with C limits at -1V depending on power rating and conductor gap. Normally varicaps are rated by C1/C2 ratios at V2, V1 like C1-30 ratio for 1-30V....