This sounds like a false dilemma. Maybe on their own, flashcards are not very effective, and on their own, mnemonic devices work well enough (for you). But this is not enough to conclude that flashcards are not useful after you adopt mnemonic devices. Maybe the additional effort (of also using spaced repetition, in addition to mnemonic devices) doesn’t significantly improve long term retention, or maybe it does.
This sounds like a false dilemma. Maybe on their own, flashcards are not very effective, and on their own, mnemonic devices work well enough (for you). But this is not enough to conclude that flashcards are not useful after you adopt mnemonic devices. Maybe the additional effort (of also using spaced repetition, in addition to mnemonic devices) doesn’t significantly improve long term retention, or maybe it does.