Vacancy ordered halide perovskites have been extensively investigated as promising lead-freealternatives to halide perovskites for various opto-electronic applications. Amongthese Cs2TiBr6 has been reported as a stable absorber with interesting electronic and opticalproperties, such as a band-gap in the visible, and long carrier diffusion lengths. Yet, athorough theoretical analysis of the exhibited properties is still missing in order to furtherassess its application potential from a materials design point of view. In this letter, weperform a detailed analysis for the established Ti-based compounds and investigate theless-known materials based on Zr. We discuss in details their electronic properties andband symmetries, highlight the similarity between the materials in terms of properties,and reveal limits for tuning electronic and optical properties within this family of vacancyordered double perovskites that share the same electron configuration. We also show thechallenges to compute accurate and meaningful quasi-particle corrections at GW level.Furthermore, we address their chemical stability against different decomposition reactionpathways, identifying stable regions for the formation of all materials, while probing theirmechanical stability employing phonon calculations. We predict that Cs2ZrI6, a materialpractically unexplored to-date, shall exhibit a quasi-direct electronic band-gap well withinthe visible range, the smallest charge carrier effective masses within the Cs2BX6 (B=Ti,Zr;X=Br, I) compounds, and a good chemical stability.