The Dawn of the Artifacts series (Dusk, Midnight, Darkest Hour, New Dawn) should be played in sequence, maybe with a run or two in between each adventure, but don't skip any. The Artifacts Unbound campaign book can and should be spread out a little more. Pick and choose the missions that fit best into your game, but mix them up with other runs. I expect if you played Dawn of the Artifacts and then Artifacts Unbound straight through, your players would get pretty tired of running into the same relics over and over again.