Even if "The Black Cauldron" wasn't an adaptation of a great book (or TWO books, really), it would still be a pretty bad film. Its pacing is off and its plotting is clumsy. It has a bland lead and some bad side characters- the witches are irritating, Doli is overly cute cartoonish (and not in a good way) and Gurgi is among the most annoying characters committed to celluloid, up there with Jar Jar Binks and every Chris Tucker character (excluding Danny in "Silver Linings Playbook.") Worst of all, the villain is boring. Some people find him scary, but I just found him a blank slate with no personality or menace, and some of the most generic motives Disney has ever produced. He's only memorable for his design, which is honestly pretty bad.
That isn't to say the movie is wholly without merit. Eilonwy is pretty good, and Fflewddur Fflam is the closest the movie comes to capturing something from the book correctly. The animation is on par with what you expect from Disney, and the more serious tone is a refreshing change of pace. That said, this movie is not even CLOSE to the darkest in the Disney canon; scary moments and demons do not a dark film make, and the terrible comic relief of Gurgi and the witches make the film significantly less self-serious than "Pinocchio", "Sleeping Beauty", "Beauty and the Beast", "Mulan" or "The Hunchback of Notre Dame." Those films had comic relief as well, but they felt more in tune with the movie and never ruined it. (I know some will disagree, especially in regards to "Hunchback", but I never found anything wrong with them).
On its own, the film sucks. But taking into account that the film is an adaptation of a particularly great series of book, it's unforgivably bad. The combination of the first two books is misguided, a clumsy mishmash of ideas that simply don't work. The elimination of several important characters like Coll, Gwydion, Achren and especially Arawn end up making the film a confused mess. The complexities of the books are undone by way of a cop out happy ending, and the Horned King is simply wrong. He's far more frightening as a man who wears a human skull than as a demon. Please, do yourself a favor- skip the movie, read the books. They're far more engaging, exciting, deep, meaningful, beautiful, funny and all around better than this.