It's made painful (for third-party tools) by the fact that the db isn't hosted on the USB partition, instead, it's hidden away in a dedicated settings system partition.
That's a win for Amazon, as it's no longer a crappy FAT32 fs, and they don't have to deal with it as far as USBMS handling is concerned, since the user/OS doesn't have access to it.
Back in ye' olden days, when they used flatfile storage to handle that, it *was* hosted on the USB partition, we could even get nested Collections going w/ the dedicated Calibre plugin, until Amazon decided: nope, not gonna happen anymore, let's move that to an SQLite db in a system partition, and update it via a REST API that, oh, surprise, takes input that eerily looks like the legacy JSON format
.
Back in the flatfile days, it also took approximately seven billion years for the UI to even register that those collections were there, too ^^.
So, again, from a performance & data integrity PoV, the switch definitely makes sense for Amazon.
(Well, after they fixed the glaring issue with the size of said partition, and they realized that people could, in fact, fill it up, which lead to hilarious breakage
).
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TL;DR: I'll take Kobo's metadata handling, and Calibre's careful massaging of said data thanks to @davidfor's efforts over any sort of "Collection" support, thank you very much
.