These are ratings based on no agreed upon criteria whatsoever. I find the irony of Amazon ratings is that on what was originally a bookseller website the thing they work the worst for is books (and other media content) since reviewers can't even agree on whether they are rating the content or the physical product.
But with a campaign book the lack of agreed criteria problem becomes even more extreme. Aside from people getting upset about shipping and such, or about receiving a defective copy (I think most of us have encountered 5e stinky book syndrome at some point), a D&D campaign book these days is also likely to get dinged by whatever cranks have found some basis for a culture wars-based polemic. But beyond all that stuff there are is a more fundamental issue: is it a rating of the adventure or the presentation in book form? If the adventure, from a player's perspective or a DMs. If the presentation in book form, did the reviewer run the campaign or just read it?
Of the listed campaigns I own and have read and/or run parts of Out of the Abyss, Storm King's Thunder, and Waterdeep Dragon Heist. From a reading perspective I like SKT the best as it has a substantial introduction that lays out the whole plot at the beginning for me to know, whereas Dragon Heist is less clear on this front, and Out of the Abyss expects me to pick it all up myself. On the DM support front SKT also has a section in the back with suggestions on how to transition from any of the then published campaigns into this one, which I thought was a great idea. SKT is also, I would argue, a stronger setting guide for the Sword Coast than the Sword Coast Adventurer's Guide. But neither of those points have much to do with the campaign itself which I have never run most of (the group I bought it for fizzled). As a campaign it seems pretty middle of the road (3 or 4 out of 5 stars), but as a book to run the campaign from 5 stars, easy (with the caveat that I only ever read the opening chapters and skimmed the rest).
Meanwhile the book presentation of Waterdeep Dragon Heist seems unexceptional, the actual adventure doesn't really seem to amount to much, and as a product it could probably support the DM better, but when I was a player in that campaign we had tremendous fun because Waterdeep is a rich setting and an urban campaign where the players own a business lent itself to lots of wild schemes and creative hijinks, and I think that is not an uncommon experience. So as a DM support 3-4 stars, as a campaign from the DM's perspective again 3-4 stars, but as a campaign from the player perspective 5 stars.
Neither of those "reviews" was my actual rating necessarily, as it was all based on memory and I haven't read all of either of those books, but the point is that based on what criteria I was rating them for I might easily vary wildly, even before we get into issues of whether Amazon sent my order too late for the first session or the book had a strong chemical smell.
