Author Existence Failure: Tolkien died before he could finish it. (But he spent years and years fiddling with the stories, so the published version is an editorial creation, assembling together various texts into a readable version that doesn't contradict itself.)
Development Hell: Tolkien began writing The Silmarillion during the First World War and it was published only in 1977, over sixty years after its first conception.
Saved from Development Hell: J.R.R.Tolkien worked on it from WWI to his death - over fifty years! - and it was published posthumously by his son Christopher in 1977. Most Tolkien fans agree it was definitely worth it.
Extremely Lengthy Creation: Tolkien started work on what would become this in 1914. After his death in 1973 it still wasn't finished. It was finally published in 1977 by his son Christopher.