High level adventures aren't easy things to put together, I've seen way too many over the years (several being from TSR alumni even) that fail to account for the power and abilities the players have at hand (I've been guilty of it myself - those levels have serious learning curves, and its why I don't play them anymore).
I was very hesitant about this module from the start after listening to the interview, and even from the interview I could sense there were going to be some railroad problems (and rule/lore snags) with the adventure. It really sounded to me like they were falling back into the 2E days of "Epic Storylines" on rails where the characters were more observer than participant (and often would never learn half the story the DM had been fed - Ravenloft, Dark Sun and Planescape were especially notorious for this).
When you're trying to fit a campaign into an X page book there's not much wiggle room beyond the main adventure path, there's probably been several chunks that have been excised just to get it to fit into the page count in the first place. I imagine there's several spots where the characters are just assumed to roll with the plot to the next big scene and there's no space (or attempt) to extrapolate if they don't - they'll just flounder until the DM figures out a way to bridge the gap to the next section.
Also, they've been dinged before for making adventures "too complicated" to run via a convoluted story path or multiple branches - I think that might have been Storm King's Thunder, but I know that probably D&D's most complicated storyline was Expedition to the Demonweb Pits, and it got utterly slammed for how convoluted it was. They don't want to repeat that experience and scare away customers - high level is complicated enough without trying to decipher someone else's storyline and half-notes about how to juggle the storyline(s).
I think what we ended up with was a storyline that could be easily followed and easily run by inexperienced DMs as their first foray into high level as long as the players are onboard for following the primary trail. It's a casual tale, not a meatgrinder. I don't think it was made for the veteran DMs like a lot of us on here who have years under our belt - we're more likely experienced enough we'd prefer to make content that specifically fits our game thus far. They probably expected a lot of groups who attempt to run this have had some game time with lower level characters (maybe up to 8th level or so, if that much) and are going to skip right to this adventure with new PCs or power-leveling current ones.
