Hey, sorry for taking so long to answer. I haven’t been on Wanikani recently.
I have a long post on my experience with Kumon in case you are interested, but it’s not focused on Kokugo.
I think the main thing to have in mind about Kokugo is that it is a material meant for natives. So it can be either awesome or awful depending on your goals and current level.
For instance, since we are at the JLPT post, I think it’s a bad choice if one’s goal is to pass on JLPT in a short time. It’s simply not meant for that. I also wouldn’t recommend it to someone with an intermediary or below level of Japanese. The lower levels are meant for Japanese elementary school kids, so they are FULL of animal sounds, 擬音語, overly informal speech, age-specific dialects (old people inside fairy tales, etc) and such. This may seem interesting from outside, but it can get extremely frustrating when you are not able to follow the texts without help and realize most of the words are not even in the dictionary to begin with.
On the other hand, it’s great for increasing your vocabulary, for exposing you to several genres of text and also for increasing your reading speed. (that’s what it’s meant for, right)
It can also make your Japanese sound more natural, since it gives you grammar as natives study it.
In general I would recommend it to people who already have a solid base of Japanese and are searching for advanced material (which can be pretty hard to find). Also, if you ever plan on working or studying using Japanese as your main language, it is great.
Sorry for the long post, feel free to ask anything in more details if you want.