People are pretty adamant that it was not intended to be used alone and should not be used alone, but my opinion is that there are worse ways to spend your time.
Now, I agree that someone who knows NOTHING about Japanese would be wasting their time. (By that I mean someone who doesn’t understand what hiragana / katakana / kanji are, can’t read any of them, and doesn’t know the most basic points of sentence construction, particles, ko/so/a/do words, etc.).
AND I also have to agree that people using diverse resources to tackle more than just kanji lessons will make more progress toward learning Japanese.
But there’s a lot of middle ground between those two poles (starting from 0, and trying to be fluent ASAP). I think some of the responses here tend to overstate the need to use other resources all the time. Not everyone has infinite time or mental energy to devote to learning Japanese, and not everyone’s goal is to become perfectly fluent any time soon. (Of course, if that is their goal, then the responses that you NEED to diversify are correct). Otherwise, I think some of the responses here tend to overstate the need to cover as much different ground as possible.
If you know hiragana / katakana, and you have a basic but decent grammar base (to where you could parse most sentences using a grammar reference, even if you can’t create the sentences on your own), then learning kanji is actually going to be a huge help to your comprehension of native material, your ability to communicate online, AND your ability to learn more grammar in the future.
In addition, for people who don’t have a lot of time, Wanikani is the easiest way to spend the time you do have, because it’s all prepared for you–you have lessons, you have reviews, it feels like a game, and you aren’t just reading through a chapter in a textbook hoping you retain some stuff after doing an example or two.
So, depending on your goal… I am not so bearish on the prospects of people who fall into the habit of only having time for WK most days. Edited to add a shorter recap that I think captures my thoughts better: If you only have time for one thing most days, using just WK when you already know a little bit of grammar does more good for you than I think people give it credit here (not just in this thread, I mean).