Most law is wishy washy. There are very few cut and dry answers in the law (If there were, we wouldn't need lawyers and a court system based on deciphering the law).
All that said, the one thing I'd add about fair use is that it isn't permission to use anything you like, but rather a defense in a legal proceeding about copyright. It's pretty much all about being able to reference copyrighted material with the law later coming in and making final decisions on whether or not that reference went too far. (IE, copying all of a disney movie and saying "What's up with this!" vs copying 1 scene and saying "This is totally messed up and here's why".)
All that said, the one thing I'd add about fair use is that it isn't permission to use anything you like, but rather a defense in a legal proceeding about copyright. It's pretty much all about being able to reference copyrighted material with the law later coming in and making final decisions on whether or not that reference went too far. (IE, copying all of a disney movie and saying "What's up with this!" vs copying 1 scene and saying "This is totally messed up and here's why".)
That was a big part of the google oracle lawsuit.