To paraphrase 2001: A Space Odyssey, "My God, it's full of words."
Mary Stewart's beginning of her Merlin trilogy, The Crystal Cave (1970), is most certainly full of words. The book follows the early life of Merlin, advisor to Arthur, while also chronicling Britain before Arthur, putting the world of Britain into its proper historical perspective.
It's this historical verisimilitude which plays havoc with Arthurian Legend. The original Arthurian tales were invented and remixed in the Middle Ages, invoking a legendary past with all the historical accuracy of Xena: Warrior Princess. The Arthurian legends were never supposed to be accurate stories. They were a story cycle as outlandish as Xena, filled with colorful characters and wicked warlords who needed to get taught a lesson.
This prequel of a book shares all the problems of the Star War prequels, telling us a story that doesn't matter with an end that doesn't really interest in us. Although this story gives us the world that Arthur was born into, it fails to give us the reason that Arthur needed to get born in the first place. Arthur and Camelot existed as a force of goodness and light against a world of selfishness and self-interest. Although we do see some selfishness and self-interest people, we are left with the impression that anyone strong enough can and will take the throne. What need to we have of Arthur?
Merlin himself comes off as something of a wet paper bag. He gets visions, but he really isn't a wizard at all. His legend comes from the superstition of others. He bounces from vision to vision, but really seems to have no opinion on these visions at all, and seems to vie for nothing. Rarely can we predict what this character will do, mostly because he doesn't do very much, and what he does do, prophesy, he doesn't control. He often comes across as far too pat.
As for Uther Pendragon, you'd think he'd be a major character, one who everyone agrees was a pretty terrific king, but instead, he's just this guy who can't keep it in his pants. Is he the last great king whose legacy means a united England? I don't think so.
So as you've rightly concluded, I think very little of this books. It's quite well written, but all those pretty words cover over the fact that its foundation are meagre. I am left pondering what the point of this story is.
The ancient storytellers had it right. The story begins with Uther's affair with Ygraine, then quickly moves to Arthur. The rest doesn't matter because it doesn't really add anything to the narrative arc.