I still can't stop but think how stretched out the Hobbit films were, for an otherwise short book. That's a sin that stains forever with me. The Lord of the Rings films, do have their own flaws, but they're fairly contained and can be debated as to whether they were good or not. I have so much trouble excusing anything with the Hobbit though.
And I hope we never, ever see anyone tackle The Silmarillion. Simply because, there will be so much improvised fluff in that would-be film, that it'd make the Hobbit films blush.
Not a fan of the ending myself but c'mon, it's been a religious sci-fi since season one, dealing with prophecies, visions and existential questions. God was always going to be a part of the equation. Unless they revealed the nature of god being some machine or a more developed civilization, at which point they would never be able to finish the show because that wouldn't be an answer but a whole new set of questions.