I like the Knaak books myself. Just like with other novels based within on lore rich universes such as Star Wars, Star Trek, Forgotten Realms and AD&D the quality of the novels will vary based on many factors. If you are going to read such novels, you have to be willing to make allowances to the authors for the creative limitations inherent in such material.

One is the format of the story itself. An alternate history format such as War of the Ancients is usually going to be more meaningful, and less confusing, to those who know the original version of events. While a story like Arthas, which is basically just an elaboration of preexisting canon, is much easier to package up as a stand alone story.
There's also things like editorial cuts (usually ordered by the publisher to reduce page counts and thus printing costs) which can leave plot holes that didn't exist in the original version. One of the more famous examples (regardless of whether you like the book or not) of this is Stephen King's "The Stand" which had almost 500 pages cut from the original manuscript (creating numerous holes in the story) to keep the cost of the book down. And in an alternate timeline story like War of the Ancients, the first things to get cut would likely be anything that could be found elsewhere in the lore which leads to that "Advent Children Syndrome".
Other things such as the scope of the story being told and approval of the IP owners can have a major impact on the authors ability to tell the story well. A story like War of the Ancients, covering numerous pivotal events in the story and numerous major characters likely has to dance around the intent of the IP owners a lot more than a simple elaboration of official cannon which can impose rewrites and revisions which, when up against a publisher's deadline and editorial cuts can greatly effect the quality of the published work.