I’ve found the original Planescape material very hard on a DM; it came across as “read, understand, and memorize these forty books” whereas with al-Qadim (al-Qadim is a problematic setting for other reasons, only complimenting the disposition here), it’s a super clean 40000 ft overview in the Land of Fate boxed set and you can then drill deeper into specific regions as you wish.
One thing that’s good about the OSR as opposed to TSR or WotC is the relentless focus on actual playability, like with something like Neverland or Hot Springs Island or Stonehell. It’s a game tool foremost, and only as an extremely distant second it’s an entertaining read. Whereas with a lot of the TSR/WotC/WW stuff it’s made for a dreaming reader with not as much an eye for practicality as I would’ve wanted.
Indies can have that same problem. Red Market comes to mind. It could’ve been “It’s our world but with these few specific differences. + here is the game system + economic system from which awesomeness will emerge”. Nope. Instead it’s hundreds of pages of alt-history that you need to read & internalize because it’s full of small details and the only source of truth for them is the prose.
That’s maybe what the core idea here is. If the only source of truth of something is a huge block of prose, that’s a problem. Apocalypse World lets the game world come through in the designs of the classes themselves. Something like the Castle Ravenloft portion of Curse of Strahd, or the pointcrawl part of Lost Mines of Phandelver, does it through encounter tables. So wonderful.
Whereas the original third-edition Eberron it’s such an awesome setting but since it’s designed in the prose format, if you wanna know that hobbits have dinos you’d better read the book as if it were a prose book because that’s the one way to learn that detail.
I found that Planescape for 2e came across that way, too. As a large amount of prose books.