Well....yes. I think it's a very good book, a great attempt at compiling most of what was going on with the story and the before-and-after. But its flaws ARE glaring, and it's not the first time it's happened with a true-crime bestseller either (In Cold Blood, Helter Skelter and Zodiac have also been accused of making stuff up.) In the 2016 expanded edition Cullen deleted the testimony of Brenda Parker (23 year old who claimed to sleep with Eric Harris) but did NOT delete his assertion that "Eric got chicks. Lots and lots of chicks." This is the one thing above all else he's taken flack for, because there's little to no evidence to support it. He also downplays the jock-bullying infamy factor too far in his attempts to blast the media for creating a false narrative to the point where he acts like it's not a factor at all. And attacking the false-narrative thing is Cullen's only reason for downplaying the bullying factor so far--Cullen admits himself that, since he was openly gay, he was bullied himself in high school and that he completely applauds anti-bullying measures in high school since Columbine. It's really weird. He claims that jock bullies should have factored more into their plan, like asking why Eric and Dylan didn't blow up a football game or something like that, and claiming that Eric and Dylan were bullies more often than they were bullied. That doesn't make bullying not a factor.
Finally, while almost everyone makes Dylan out to be the sensitive one and the "follower" (Brooks Brown included--it was the first thing he said to the media in 1999, practically), Cullen goes somewhat overboard in making Dylan out to be an innocent, even when he does cite the thing Dylan wrote in 1997 about planning to shoot up the school with some girl as his accomplice. He uses language like "what a sweet, loving kid" to describe Dylan and says that "He was not a man of action. He was conscripted by someone who was." Conscripted?!?
The problem is that the people who hate Cullen the most are online armchair-detectives and Columbiners, who are not only too hard on Cullen's book but a bit crazy themselves, and not just the goth girls who are in love with Eric and Dylan and write disgusting fan fiction about them. For one thing, they try to claim Eric wasn't a psychopath and their two most frequently cited reasons are flatly retarded: 1)that Eric's extensive hate-filled rantings online and in his journals are "acting" since he knew he was going to die--who puts that much effort into an act? 2)FBI guy Dwayne Fuselier is in no position to diagnose Eric after the fact as a psychopath because Fuselier's kid was at Columbine on 4-20-99. They also like to dig up incidents that might not have even happened, like the "ketchup tampon" food fight (which has never been proven) or bringing up names like Rocky Hoffschneider, a by-all-accounts psycho wrestler guy who graduated the year before the massacre and who was allowed to park his Hummer in the faculty parking lot and basically get away with whatever he wanted.
My final conclusion about it is that it's somewhere in a grey area....I think they had problems, and that the school environment likely made those problems worse. Not an either/or thing at all. It's just difficult to determine how much of a factor it really was.