If you look at the most played in 15+ aff is at the top lock spec.
4th best dps on raider.io
https://raider.io/characters/eu/twis...her/Sjel%C3%A9
you can even watch him play if you want to learn instead of complaining and asking for buff
https://youtu.be/1IWbEDspvmY
It's also so much easier to play then destro, destro is extremly strong AoE but you need to play around CD well and you need a tanks that does not kite. 2 thing that basically don't exist in pug right now.
Aff STS build is just super strong in m+, there is no other way to put it.
Ele was extremely strong in season 1 and no one was playing it for like 2 month.
As for boss fight:
-Tarragrue is a Giant joke... count it as ST if you want
-Eye is not ST, it as an add that as decent uptime in P1 and P2 is cleavable, you can dps both P2 eyes at the same time in hero and in mythic, that's why top parse for lock on this fight are destro.
-The nine is 2T for basically all of P1, The hardest part is 3T (This fight is a joke on mythic)
-Remnant as the orbs that need to be dps down and Aff can get shard out of them. (This is one of the best Aff fight this tier btw)
-Soulrender is an AoE fight. Aff can do really good dmg playing Stachel and STS build on this fight (top lock parse on this fight are Aff)
-Painsmith is not a good Aff fight as you need burst on the balls, this is the fight were you need to play destro.
-Guardian is ST but you can play malefic wrath on it.
-Fatescribe as a lot of adds, P1 as tank add and P2 as 2 target cleave.
-KT is pure AoE fight, the reason Aff is not played is because it's one of those hold dps fight. in hero playin WoC on this fight is very strong.
-Sylvanas is AoE, this fight was all about P3 dmg on fist kill but it's going to be less the case in a few week.
That's 2 fight that are pure ST. Tarragrue and Guardian. With Tarragrue being a bonified trash.
Look, Is AFF the best spec to play in SoD? No it's not... Is it good enough to play on basically every fight even in mythic? yes it is.
Last tier was aff, this tier is demo/destro.
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My spec is not the best in pure ST and in Sims, situation that I face basically never. better quite the game...