I only tried WK briefly, but I found it mystifying - the whole system where reading a word gets decomposed into a multi-step process (kanji -> radicals -> names for radicals -> mnemonic linking radical names into character meaning -> character meanings -> word meaning).
I mean - if one's goal is to reach fluency, then eventually one will need to discard all those intermediate steps, right? But the app didn't feel to me like it was headed in that direction. To be honest it felt less like an app for memorizing Japanese words, and more like an app for memorizing radical names and mnemonics linking those names to character meanings (i.e. stuff that the WK authors made up). I am way off?
Learning the radicals is one of the most useful things you can do when learning kanji. With just knowing the radicals, you can sometimes at least know what a kanji represents, even if you've never seen the kanji. For instance, if a kanji has a fish radical in it, it's very likely a kanji for a fish. If there's a clam radical, it could be a clam, or related to money. If there's an animal radical, it's very likely an animal kanji.
Similarly, knowing the individual kanji and readings helps learning combination words, and guess at how to read them, when can be helpful sometimes when a word is written in kanji, when it's usually not. You know the word, and one of the kanji in the word and can use the radicals to guess the readings and know the meanings and realize the word.
The radicals and mnemonics are useful for bridging the gap between learning a word and truly acquiring it. Think of it like learning how to read in English: when starting out you have to sound out every letter, recall the weird rules about how sometimes "ough" can make an "oo" sound (or was it an "off" sound?), use your finger so you don't loose your place etc. But eventually with enough repetition your brain sees the jumble of letters that is the word "throughout" and immediately knows what it says.
Everything that WaniKani does is just to help you get from seeing "日本語" and it meaning absolutely nothing to you, to seeing it and immediately knowing what it means. The radicals and mnemonics are all secondary, and don't need to be remembered.
When you think about how you read English text, or text in any language really, you don't decompose all that much during normal reading. For example, if you look at the word "house", you don't read it like "H-O-U-S-E-house" like a child learning to read would, because you have learned to recognize "house" immediately.
But even then, it's still useful to learn how the different bits and pieces of language fit together (etymology, root words, and such) in order to parse new words that you encounter for the first time. Even if you have never encountered the word "quizzacious" before, you can see the root word "quiz" and can make an educated guess about its meaning.
That's true for English roots, but for the WK radical names we're talking about here, many of them are just arbitrarily chosen words. E.g. they call 歹 "yakuza" and 各 "kiss", etc. There's not necessarily any connection between the mnemonic words and the radicals they represent.
(And even for the remaining cases, a kanji's radicals don't generally tell you much about the character's meaning the way English roots do. At least not outside of a few cases, like 言 or 魚, which don't need to be memorized because the radicals are already basic kanji.)
I mean - if one's goal is to reach fluency, then eventually one will need to discard all those intermediate steps, right? But the app didn't feel to me like it was headed in that direction. To be honest it felt less like an app for memorizing Japanese words, and more like an app for memorizing radical names and mnemonics linking those names to character meanings (i.e. stuff that the WK authors made up). I am way off?