Affliction does pretty great in Single Target, it might be a bit behind Destro but its much more mobile and in M+ that usually brings it to about the same point.
In AOE, Affliction relies on seed spam, which costs shards, and packs large enough to see value out of soul flame. Because aff shard gen is RNG off agony tics, its shard gen sucks at normal pull sizes. It sees play in Academy and sometime NO because the pull sizes are big enough to support fast shard gen and soul flame value.
Aff is pretty weak at 2-3 targets and meh at 4 because of the shard gen issue.
In dungeons where the pull size is smaller, which is basically the other 6 (HoV is debatable), it’s really hard to gen enough shards to seed spam.
Comparatively, Destro has slight stronger albeit more stationary single target, fantastic 2 target, and is comparatively meh at 3-4 and begins to excel at 5+ just like Aff. Unlike Aff, it’s shard gen is very stable and deliberate, so you are unlikely to be randomly shard starved on a pack.
Demo is the only spec of the 3 that can really burst AOE quickly and frequently on 3-4 mob pulls, so for keys below ~17 I think it’s the much more practical choice. This late in the season keys below 17 are going to be filled with 410+ilvl players and if you are trying to build up RoF stacks or get a bunch of seeds out, you’re going to have a hard time keeping up with the bursty hunters, DH’s, shamans and monks.
Demo’s down side is that it kind of sucks at ST outside of it’s burst, but in the lower keys the boss might not really live long outside the burst anyway.
*and that Felguard requires manual positioning on some bosses for felstorm to hit, like 1st boss of RLP or 3rd boss of SMB.