Comparing slipbox vs. flashcards
Spaced repetition flashcards are great, especially when you write the cards well with implicit references to other cards. However, they may not be the only effective way to do massive learning.
The bare slipbox as memory trainer
A slipbox/zettelkasten naturally causes a degree of spaced repetition as well as self-testing, and helps you properly understand what you're learning because you connect it to other things you know. Thus it kind of follows memory science.
Sönke Ahrens highlights some contrasts with flashcards, which follow such a rigid format that you're not so free to explore the ideas as you write them, or after.
Just writing flashcards is an impoverished form of notetaking – half the purpose is practice at "getting the gist" and making connections, and you can do this faster without the constraint of needing to formulate your notes in a flashcard-able way. (An extension: WET your slipbox)
The "testing" occurs when you write a note in your own words and later when you extend and elaborate on it and refer to it from other notes, which is also "spaced repetition" although without an algorithm.
Revisiting to elaborate on a note is like using those non-standard open-ended SRS cards where the front side prompts something like "think of three reasons the efficient market hypothesis can be misleading" and allows your mental answer to differ each time, as the point is just to get you thinking anew.
Such SRS cards cause really good learning but unfortunately can feel artificial, like being tested in school on a topic you don't currently care about, and it's possible to 'fall off the wagon' of regular testing, whereas every time you work with your slipbox, you're doing it out of a compelling personal reason on that day and that hour on that topic, so it's a habit that self-repairs.
Flashcards pass out of your life eventually
(Or: Keeping all writings in SRS can give you a "graveyard of notes".)
A surprising fact about SRS: flashcards are not permanent! In the end, every card will effectively disappear from your life because the review delay grows to decades.
It's supposed to be fine because the idea is that at this point, you remember the answer anyway.
I submit that you actually don't remember it in everyday life – only when you enter those specific situations that trigger this memory – which may be almost never. It's not that you remember, it's that you can remember.
And thinking about it, encoding an idea in a flashcard gives you very few entry-points to revisiting that idea so you can develop on it.
Isn't it sad to have put in so much effort crafting that perfect phrasing, and you never see it again?
To put my thesis another way: when I used Anki in the past, I ended up with a habit of actually looking up the Anki database for something I wrote in the past – facts, figures, a good turn of phrase etc. I was accessing it almost like a slipbox!
But Anki is not really made for such browsing, and you can't hyperlink directly to other cards, and it deals with flashcards as its atomic unit: you get to make one card front and center, onto which you can hang supplemental information (as extra fields), whereas I'd prefer it the other way around, because the same supplemental information could be relevant to multiple cards at once.
In a slipbox with integrated flashcards, you can do this! (My solution: github.com/meedstrom/inline-anki)
Usage
Think of the slipbox as the primary product of your knowledge work and any flashcards as supplementary. Both have science behind them (flashcards have stronger science, but it's about a very specific kind of benefit), but one should be prioritized so you can get above a critical mass of connections, and the slipbox being less strict enables more kinds of connections, more kinds of knowledge, and even speculative notes – so important for exploration!
Plus, it's ADHD-friendly notetaking due to immediate relevance each time you use it, and because half-finished notes still integrate into the system.