My overall feeling is that this is a reasonably nicely told story that shouldn't have existed.
Up until this point, the Twelve's existence was ambiguous. They were the gods that the population of Eorzea believed in, with no tangible proof and plenty of past implication and speculation that they might not be real at all. It could have been left at that, especially once we had already found the real gods (or closest thing to it) that had physical influence over the planet.
Are we now going to have to confirm and deal with the existence of the kami, Azim and Nhaama, the Mol tribe's elder gods, the real Mriga and Manusya and whatever gods we find out there too? I assume and hope not. And if those can all remain ambiguous, then why did the Eorzean pantheon need resolution?
On a separate note, the ultimate irrelevance of the Watcher feels odd. His entire involvement in the story and addition to the list of the Twelve seems to have been to act as a red herring for what we expected as the final boss.
Also, as a few others have similarly commented, I didn't really like the "here are the true mortal identities of the Twelve" thing but did appreciate learning that Hythlodaeus had a competent second-in-command running the show, because of course he did.
But anyway. Godsspeed, Deryk; may the eternal wind carry you onward, and may we meet you on the road. Maybe hanging out with a yellow bird and a little robot.