Got the warlock and druid ones back in May, I was interested in the lock and guardian druid ones, but after I finished those I just did the other druid specs in spare time. The resto druid one was the biggest effort because it was one of the poorly tuned challenges and I really dislike all of the resto druid artifact skins.
I almost wanted to put in the effort to level up my dk and get those, but I couldn't find the time to squeeze that in.