For me personally, a huge part of the value of WK for me is that it sort of taught me how to learn Japanese. Or at least, it gave me some tools that were enough to help me get started reading Japanese in the wild, and my other learning followed from there.
I started WK with very little knowledge of Japanese outside of how to read hiragana and katakana. I knew nothing about on and kun readings, rendaku, radicals, semantic-phonetic composition, stroke order, pitch accent, different forms of adjectives, transitive/intransitive verbs, する verbs, or even what verbs looked like in general.
WaniKani was my introduction to every single one of those things. A lot of them took me a lot of exposure before I started to get the basic idea of them! Of course, I’m using a heavily modified version of WK with a lot of scripts that add extra information to reviews, but I’m still learning that extra information following WK’s structure.
By the time I started learning grammar and trying to read more than just occasional words in tweets, I had some idea of what I was looking at when I encountered a new vocab word. Even if I didn’t know the kanji, I could sometimes make a guess at the reading based on the composition of the kanji, and if I couldn’t, I could at least look at the kanji and remember the shape of it because I could remember the radicals. This is useful if you’re trying to recognize a name that contains kanji you don’t know, haha.
I was able to go right from WK to pre-learning vocab in Anki in preparation for reading a specific book. I’ve learned a lot of words at this point that contain kanji I’ve yet to learn in WK, and although I can read those words if they have furigana, it’s hard for me to read them on twitter or in magazines or on TV without furigana.
I do need to spend extra time studying the kanji specifically if I want to remember how to spell the words I’m learning. In the future, I’m going to be responsible for structuring my own study there (I’ve already started working on memorizing some kanji I’ve encountered that aren’t in WK), but that takes work to set up, so I’m happy to let WK do a lot of the work for me.
I guess it probably depends on your goals how “efficient” WK is or isn’t. My main interest is learning the language so that I can watch wrestling and read tweets and interviews, but my medium of choice is one that can be enjoyed without words, so there’s not really any reason why I have to become fluent as soon as possible, as long as I’m making steady progress. I also want to be able to read manga in a bunch of different genres, play video games, and watch unsubtitled anime. I don’t mind learning unusual or obscure vocab, because I’m interested in learning the language very broadly. WK also taught me a genuine love of kanji, so I love learning kanji just for the sake of learning them.
I just started learning Japanese a little over a year ago, and I’ve only been working on it diligently for the past seven months, but I’m already at a point where I can effortlessly read some tweets, which is far more progress than I expected to have made. Would my studying have been more efficient if I hadn’t used WK? I doubt it. I wouldn’t have known even where to start learning kanji if I hadn’t tried WK. I’d probably be completely tied to Yomichan (if I’d even discovered it without these forums), and would be absolutely helpless trying to read from a print magazine or from the TV screen with no furigana. I’d also be very annoyed and frustrated with kanji, probably, because I had to spend a lot of time with them before I started to love them.
I guess to make a long post short, I am definitely in favor of doing your vocab lessons, and at least for me personally, I’ve had better luck learning vocab through WK than I have either through a textbook, or through mining words with unknown kanji from manga with Yomichan/Anki. I don’t mind learning “useless” words or uncommon kanji because I genuinely really enjoy learning kanji and any new words, at this point, and even if I don’t see them in native media, they’re often valuable to me for what they teach me about the components of other kanji, or stroke order, or rendaku patterns and such in the case of vocab.