shadow doesn't have aoe choices period. the only aoe tools we had in WoD (which were already embarrassingly bad for 50%+ of the expac) are being removed.
since shadow's single target is being toned down to median-ish, I highly doubt shadow will have a niche where they actually make up for the spec's mechanical inflexibility. the only possibility would be something like another large buff to voidbolt's damage so our small-cooldown target swap/priority damage becomes significantly better than the field (with 4t19). however, this is a hard niche to actually make relevant because you can just add more and more dps with meaningful burst cooldowns to cover for it. or, of course, a 5 minute vaelastrasz style fight where we're in execute the entire time and can cheese it with STM.
the gratuitous wonkiness of how multidot and voidbolt refreshes work really just show that blizzard doesn't care about what playing a shadowpriest actually feels like. this is the most egregious example of being hyper button bash super short gcd locked for mediocre results that any spec has ever been. that our multitarget efficacy has such a low target cap means we have to compare our tools to the super OP cleaves that are in game - and unless arms wars or destro warlocks are undertuned memes there's no way our 3 target spread dps will be worth bothering with compared to very well controlled free cleave (along with meaningful burst cooldowns, etc).