It's been a long road. The Beating of the Aquiline Heart has roughly twice the content of Under the Eye of the Tempest in terms of encounters, at least by my gut feeling (I didn't count). There are tons of monsters, tactics that needed to be changed because of changes to spells, and more. Most significantly, wading through tons of minions does not work as well in 5e's bounded accuracy and would case combat to drag on too long. The encounters have been made more focused, with fewer minions and getting the heroes to the battles where they are most needed with the explanation being that the rest of the army is making a hole for the heroes to head through. Many of the encounters in the first wave of the battles in the Opaline Wastes also feature the allies the heroes have met along the way rather than having them be optional requests for aid.
Also, Roll20 has been pushing their update to their new lighting system and getting the third-party publishers to update their content. This has created a lot of extra unpaid work on my end and kept me from working on new projects.