This is a fair criticism, and ultimately comes down to personal perspective, but for me this is one of the most crucial and appealing things about Boogiepop Phantom, at least the telly version: it tells its story from the many fractured perspectives of characters who are often bystanders, and never a single nor omniscient voice that has and knows the full story. It's not Baccano!, because we can never quite piece together the whole picture with the overlapping narratives, and it isn't Rashomon, because subjective perception is not what's at question but rather subjective experience, but that's what helps make it damn near unique in its telling.
I can see how this would be highly frustrating for some, but I suppose I'm coming at it as someone who has never been particularly concerned about loose ends of plot as opposed to character. The focus on individual stories and perspectives threaded together kaleidoscopically and grounded by an authentic view of adolescent experience forms the fundamental basis for the show, and is what makes it successful (or not.) I don't believe it does or even should rely on knowledge of the light novels to be "complete," as it were, for that reason.