I think there are two reasons why Pooh works better as a "book report" than Nemo does (though Nemo technically isn't a "book report" but rather a "sequel that rehashes the movie" a la Frozen Ever After).
First off, it's easier to adapt a featurette into a dark ride than it is a movie. Winnie the Pooh and the Blustery Day has a more simple plot than Finding Nemo, and the ride's use of a narrator and storybook pages help fill in the gaps. I think this is the same problem with Disneyland's Monsters Inc ride - the film's plot is really complex, and it doesn't translate super well into a "book report" dark ride.
Number two, Pooh does more to actually involve riders in the story. Gopher and Tigger actually talk to us, we get to bounce through the forest with Tigger and float around in the Floody Place... heck, it makes no sense for us to somehow travel into Pooh's dream about Heffalumps and Woozles, but nobody cares because it makes for a great dark ride scene. Being a "bus bar" as opposed to an Omnimover helps, as it allows the vehicle to actually travel through the scenes rather than just have the scenes play out in front of them a la Nemo.