I haven't read the book so feel free to ignore anything I say about it, but based on how other people (even fans) describe it and the excerpts posted in thread, it really strikes me as a textual equivalent of how little kids play with action figures -- by throwing together lots of disparate toys and props from franchises they like, making up nonsensical stories about them all doing battle with each other or saving people in their ad-hoc shared world, and generally acting like if you take a few pre-existing things that are awesome and mash them together, the result must obviously be multiple times as awesome, right?
That's fine and great to do as a kid because it's just fun and play and ephemeral and it's how we first learn to use our imaginations and no one pretends it means anything on any level. But kids do this not because it makes for a better story or because they're trying to come up with a consistent world in which to explore anything deep or meaningful ideas, but because their imaginations aren't developed enough yet to know how to extract the substance and salient themes from the stories they're enjoying and synthesize that into a new thing. All they know how to do to vicariously recreate some of that awesomeness they love is by dropping the actual thing into their own play space and mimicking it as-is.
Like, if you think Indiana Jones is an awesome character and want to play with Indiana Jones as a kid, it's probably because on some level the theme of a dashing archaeologist who travels the world and has adventures and saves people from danger, but not for personal fortune or glory but just to further the world's understanding of ancient cultures, is really fun and resonates with you as a device for fantasy storytelling. And if you later go on to want to write your own story in that vein, you might take some elements you like (just as Lucas took from the Saturday-morning serials he was influenced by) but hopefully you'll also have enough world knowledge and some sense of other ideas you want to explore that you can create something that is largely your own -- just like the studios that made the Uncharted and Tomb Raider games respectively were clearly heavily inspired by the character and genre of the Indiana Jones series, they didn't need to rely on evoking the player's nostalgia for Indiana Jones in order to stand on their own, and they in turn put their own spin on the treasure-hunter-adventurer genre.
But as a kid, you don't know any of that. You don't know how to think about genre conventions or how to convey characters' internal struggle or establishing conflict or how the world still has to have its own internal logic and how the series explores themes of western imperialism and whether Indy's efforts are ultimately justified or fruitful. All you know is who Indiana Jones literally is and that Indiana Jones is awesome. And if you want to write a story that is also awesome, I guess one way of doing that is to invoke things you already think are awesome and drop them into your story.
But it's hard for me to think that's a praiseworthy or respectable way of writing. And not because of the trite debate about popcorn entertainment vs. high art -- Indiana Jones is popcorn entertainment but does it brilliantly and stands on its own without having to push the nostalgia button a hundred times. Rather, it just screams out to me, "I have no idea how to write or convey any ideas or emotions or character development or construct story using my own ability to craft prose; instead I have to latch onto a plethora of other, better creators and writers and hope that some of their creative talent rubs off on me and the audience won't notice." Is there any actual thematic significance to why all of these properties would need to exist in the same world in the first place? Because if not -- and the impression I get is that there isn't -- then it just strikes me as people getting high on their own childhood nostalgia by reliving the way we played with action figures as children.