Agony is useless for trash pulls except as a shard generator. There's a huge tradeoff between single target for the bosses and everything else that AC is a much more viable option than Writhe or SL. You can snipe shards with UA but that's much harder than using DS because it's just easy to switch targets that are about to tie and tap them with DS.
Sow the Seeds is absolute shit and not even worth bothering with in any circumstance. Seed of Corruption itself is very weak AOE, the only reason aff did strong AOE in Legion was Soul Flame chains. I have no idea why they think that afflocks need to be chained to a cast time and damage to detonate requirement when SoC is so weak to start with. PS has such a long cooldown it's no use in dungeons and could even end up being like it was in Legion, more useful as a single target booster since aff's single target was so mediocre (to keep down it;s multidot damage). Vile Taint is the go-to, or will be until they nerf it several times more - probably lowering the base damage and removing the synergy with the Darkglare. I can absolutely see this happening.
Pretty obvious they just want AOE to be aff's weakness. Shame it;s so critical in M+