This is why for use case (2) with any non-trivial content, the design of the cards is important, and there needs to be desire for the learner to put in some effort to connect it mnemonically. I started a couple music interval recognition decks but stopped them because I quickly felt it was too hard to learn through only basic exposure and spaced repetition, and that combined with feeling like learning them was hardly any benefit in my life made going through those cards grueling.