It was just a Beta after all, two whole factions were completely cut out, and even the Hive and the Fallen must have more different types of enemies we haven't seen yet. Story Missions aren't really meant to be as action packed as Strikes anyway. The Strikes are where the really cool missions are, fighting more than one faction at a time and mixing it up with different sections (Chaotic Hold off, Spider Tank, Sepiks Prime being Devil's Lair).