Young adult fiction has at its heart an intent to instruct, even if it says it doesn't. Children teeter on the brink of adulthood, moral dilemmas are imposed, the question of good and evil is posed and dismissed. This is often the least satisfying part of young adult fiction. Even Harry Potter, which is thoroughly enjoyable on many levels, suffers from the evil-is-evil definition of evilness. (Although, to be fair, she does underline the fact that bad people love their children too.)
Voices is definitely didactic on some levels, but Le Guin's questions are different. Even when violence is justified, can it be avoided? Especially when violence is justified, should it be avoided? How can cultures with differing values co-mingle?
The characters here are definitely not as strong as some Le Guin has created, especially the narrator, but she still has a nice ear for the way people speak, and what it means when they don't.