Apparently Stormrage isn't really that bad. I haven't read it of course, just what I heard.

The rest of Knaak, though...

Knaak is one of many writers that suffer from what I like to call Advent Children Syndrome. They write their stories and work their plots assuming the reader already knows a vast amount about the setting and backstory. This effectively limits your fiction works solely to fan boys who would already like it no matter what it is (see: Final Fantasy Advent Children) and leads to choppy reading with huge plot holes that confuse the rest of the normal people that try to read it.

I'm saying this as a very avid gamer. All you fan boys out there know I'm write when I say this too: even those of us that do know about the setting still recognize the lack of effort that goes into these stunted stories. It does make a difference.

Trust me, Night of the Dragon is worse. My inner editor screamed in pain the whole time I was reading that atrocity. -_-;

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.