I don't expect Disjunction shells to work against Efreet/Shaitan/Djinn fortifications. Sure it's incredibly hard to block a Disjunction on a large scale, but it's rather easy to block projectiles with magic, and these empires have spent millenia fortifying their main city. It probably has literally every ward possible, especially on its fortifications. A mere enthropic shield has a 20% chance of deflecting incoming projectiles, no save allowed. Then there's wind effects, force effects, weird repulsion effects targeting specific materials like "metal"...
tl;dr = I don't actually expect the shell to connect with its target, which means that we risk wasting those shells.