Maybe I had my expectations too high because of Frozen Wilds. That felt like a full game in and of itself - both from a story and a game play perspective.
For this DLC though it kinda felt like a combination of sidequests that were originally planned for the main game but didn't make the cut. That being said, WELL worth the $20 just like you mentioned. I'd have happily paid that just for the Horus battle (and I definitely couldn't see them making that battle work on PS4).