The book itself tries to do too many things at once, often rushes through scenes and events, and never quite gels. I'll tell you my theories and observations, and you can tell me that I'm wrong.
Before I being, we must consider that Dragondrums was under development at the same time as The White Dragon. The two books clearly had an influence on each other. Piemer winds up on the Southern Continent as an explorer with no explanation, Mirrim has impressed Path as if you should already know, and this girl Sharra shows up. Clearly Anne knew something of the outline of each book, with The White Dragon getting first development.
The most mystifying part of Dragondrums is that Menolly is NOT the lead character.With her wild popularity, you'd think that she would be the lead in the book again. At one point in the process, I am sure that she was the lead. Dragonsinger clearly promised that she would get involved in more interesting Harper tasks, letting her journey as part of being a journeyman. The book is about all those others things that Hapers need to do. You can still see the original story arc, where Menolly finds Oldtimers taking gems from mines, tracks down rogue fire lizard eggs to Meron's hold, Piemer gets taken south, sails to the Southern Continent to rescue Piemer, gets lost with the Masterharper, falls in love with Sebell, and then what? I really don't know. The getting lost part is clearly referenced in The White Dragon as if that were another book in the series, which it wasn't.
My guess is that editorial heads prevailed. The biggest issue was that of Sebell. If he and Menolly were to have sex a boat, and Menolly was only fifteen, that would cause publication problems. Fifteen year olds don't get in going on a boat, even with fire lizards as an excuse. At a minimum, Menolly would need to be eighteen, and story would need to shift to three years later, which happens to be the exact time shift of the story, three turns after Dragonsinger.
In addition, Menolly clearly needed to spend more time at Harper Hall learning. She needed to get her voice up to snuff. In addition, her job was clearly to write songs. Why would you put your most important song writer, someone who could influence the entire planet, into harm's way? That further emphasises the need to time shift the book.
The original concept still could have worked as referenced in The White Dragon. My guess is that there are some details that didn't work any more. For whatever reason, perhaps because she's not out of the YA age range, Menolly got relegated to a secondary character.
The only other Harper of the appropriate age is Piemer, and so Anne falls back to him, presumably adapting Menolly's story to his story, while still keeping Menolly around to be part of other bits of her story.
The first half of the book seems well paced enough and full enough of detail, but the middle book just goes astray. The storytelling switches to this point-of-view style that is all see this, does that, and very experiential. There's nothing like it in her other books and I just don't think that it works.
The back half of the book feels rushed. We wander through so many scenes in a summarized manner that I'm positive that Anne was behind writing in a time crunch, had her novel going over the contractual word length and needed to condense to fit in everything from the outline, or both. I vote both.
To be honest, much of the book feels like backstory for The White Dragon.
Of all Anne's books from the 70's, I think that this is the only one that lost me. Halfway through and I found myself losing steam. Dragondrums may be among the most favored of all 1970's fantasy, but I can't say that it's among the best.