Flashcards really are useful only when you want to associate 2 things to each other (for example a word and its translation) and might not be the best way to build an organized knowledge of a subject.

That like saying that computer are only useful when you want to add 0's and 1's. There's Turing completion for computers. In a similar way you can break most knowledge down to atomic units. Often that means that you have to think about the knowledge in a more detailed way.

A question to the community: Do you really believe as much in spaced repetition/Anki as the post suggests?

If I remember right something like 12% use it. On the other hand more than 12% did use it but don't anymore. On the other hand those people who speak about Anki, are the people who believe in it.

However while I do believe in it I think there's a huge room for improvement through things like automatic creation of Ven diagrams based cards.