Critical Research Failure/Dan Browned: Ridley Pearson has gone on record stating how much research he has done on how the park and rides operate...this apparently does not extend to the characters themselves, however. Depending on the kind of Disney fan you're dealing with, they're either going to see liberties that Pearson took as Acceptable Breaks from Reality or as entirely unfeasible.
Most Disney fans that don't like the books make the argument that Pearson hasn't done research into the spirit of the park itself, including the fact that Walt Disney didn't even want to use holograms in any of the rides or the park itself after being approached by a company that created them, and Imagineers have continued to stay away from using them ever since. So technically the DHIs should not even exist. However, Disney Research, a R&D division of Disney that develops technology advancements that could be used by other divisions of the company later on, has developed means for people to touch virtual objects, including holograms. Disney parks have also utilized special effects often confused for holograms, the most well known being the Pepper's Ghost illusion, used in attractions like the Haunted Mansion and the Tower of Terror.
The park is never completely abandoned after it's closed, not even by characters come to life. There are cleanup crews, Cast Members double-checking rides to make sure they are working and safe, and more. If there were characters running around and chasing the DHIs, wouldn't security have noticed by now?
There are a ton of characters that walk around where they're not supposed to be, like the Fantasia Brooms in the Voyage of the Little Mermaid show and the Country Bears in Mickey's Philharmagic. Not to mention Maleficent and Finn dressed as Aladdin fighting in Tomorrowland in broad daylight with tourists watching. There's always a Coordinator or Manager watching the action somewhere and ready to report to the higher-ups in any department. This kind of action qualifies for instant termination for whoever's in those costumes. Even if it's technically the characters themselves, it's strange that, beyond Rule of Cool, why nobody mentions how odd it is that it happens in the first place.
The preview for the seventh book says there were six weasels with Judge Doom. There's only five in the movie. Either a new weasel has joined the Toon Patrol, or this is a very glaring version of this trope.
The errors in attraction names. "Fantasmic!" is repeatedly referred to as "Fantasmics" and Buzz Lightyear's Space Ranger Spin is referred to as Buzz Lightyear's Astro Blasters, which is the Disneyland version of the ride.
Overlapping with Spell My Name with an "S", Sabor is always called "the Sabor," no mention of her gender is brought up, the pronouns used are "it, its," and in a nod to the original Literature/Tarzan, is called a lioness rather than a leopard.
Darkness-Induced Audience Apathy: By the sixth book some fans have stopped reading because they feel it's gotten a little TOO dark, even by Disney standards. A blood sacrifice resulting in the death of a minor character by his best friend will do it.
They Wasted a Perfectly Good Character: A common complaint of the series is that despite being set at Disney World and Disneyland, most of the Disney characters outside of the villains only get cameos or a Big Damn Heroes moment at best. And then The Insider and Disney at Last happened.
They Wasted a Perfectly Good Plot: An adventure in the Magic Kingdom and the other Disney Parks after dark with the characters coming to life and fighting Disney villains? Sounds awesome! Until you get to the bland characters, too much focus on the technical aspects, poor characterizations of characters we already know and love...how long until Jon Favreau's Magic Kingdom movie come out?
On a more specific note, the ending of The Insider depicts Tia Dalma preparing to resurrect Maleficent, but the latter only gets two post-ressurection action scenes before being killed again, this time for real.
Wangst: Depending on your level of tolerance (and how close you are to the target age of the series), you might find Finn's concerns of his normal life (bullies, high school, love triangles, disapproving parents...) on top of fighting Disney villains to either be par for the course or utterly unbearable.
The Woobie: Dillard. The poor guy's been totally abandoned by his supposed best friend and he gets killed in the sixth book all because he wanted to be a DHI.