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.note Usually.)
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 and four years after his death.
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.
In addition, Amazon is creating a Lord of the Rings television series. Details are currently sparse, but it is known that it will take place at least partially in the Second Age, during the time of Númenor, so it might be that the Akallabêth will be at the very least part of the adaptation.
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.
In some stories printed in The History of Middle-earth, Tolkien noted that Ungoliant was killed by Eärendil on his voyages. He later dropped this idea and had her possibly devour herself.
Ungoliant was originally a primeval spirit of Darkness, whose origins were completely unknown, though presumably a creature from the Void. This is changed in later versions of The Silmarillion, where it is more evident that she was one of the Maiar that served Melkor until she deserted him.
The first versions of Lúthien and her tale are very different from how the story finally shaped out to be. Originally, she was a blue-eyed blonde dressed up in white, Daeron was her brother, and Beren was an elf, Sauron was a huge cat, she didn't die but walked into Valinor over Helcaraxë to meet Mandos, and so forth.
In some drafts for the Akallabêth, Míriel fell in love with Ar-Pharazôn, married him voluntarily, and gave up most of her power to him.
In the earliest drafts, elves had a very different form of Immortality - they shrank as they got older, becoming Miniature Senior Citizens. This was an attempt to explain how the old Norse concept of human-sized elves gave rise to tiny Disney-esque fairies.
Word of Saint Paul: Christopher Tolkien's commentaries and edits on posthumous works. Also his version of the war between Doriath and the Dwarves, which his father never got around to writing. Specifically, the only complete version of the events existed in Tolkien's Book of Lost Tales, the very first version of what would become The Silmarillion. This lead to continuity issues as Tolkien re-wrote and updated his stories.