I did the Demonology Warlock intro quests and artifact weapon.
I will say that I had a blast. The demo lock quest is pitch perfect with the class theme. The order hall full of your army of demons is awesome. I think this was my favorite one so far.
However… I neglected to read up on the extensive changes to the demonology spec that happened in Legion. I went into the quests not knowing what any of my buttons did. As a result, I was doing a whopping 6k dps through most of the questline. When I popped cooldowns I think I broke 10k briefly.
And yet I made it through just fine. Which begs the question – is it possible to fail these questlines? How badly would you have to play to get stonewalled by the quests?