There are some discussions about this on Reddit including these:
https://amp.reddit.com/r/LearnJapanese/comments/2h1ag4/is_tae_kims_learning_japanese_app_for_android_a/
https://amp-reddit-com.cdn.ampproject.org/v/s/amp.reddit.com/r/LearnJapanese/comments/3hma3w/tae_kims_complete_guide_vs_grammar_guide/?usqp=mq331AQCCAE%3D&_js_v=0.1#referrer=https%3A%2F%2Fwww.google.com&_tf=From%20%251%24s&share=https%3A%2F%2Fwww.reddit.com%2Fr%2FLearnJapanese%2Fcomments%2F3hma3w%2Ftae_kims_complete_guide_vs_grammar_guide%2F
To be fair, some of the accusations come from the creator of Imabi, so I’m sure they’re biased. On the other hand, the creator of Imabi clearly is very knowledgeable.
Not satisfied with his word for it, though, I did further research and found this from Tofugu, which, of course, I trust (more than anyone else):
If you scroll down to the Tae Kim review, it states, “Some grammar explanations may contain errors.”
I initially found this to be alarming, but I recently reread the chapter on how to learn grammar in The Art and Science of Learning Languages and the authors stated that all grammar books contain errors to a greater or lesser extent. So, maybe I shouldn’t be so worried about Tae Kim. While the creators of the other resources would surely argue that they contain no errors, surely they do as well.
Truth be told, while it has a great reputation, it really is a reference work and not ideal for teaching yourself grammar from nothing (or, almost nothing, since I did do Textfugu, so I have some basics already). Ultimately, as I’ve explored in another thread (Most concise grammar resource), I think it may just be better to find something more concise. I’m thinking Imabi and DBJG will be of use in the future to get a deeper understanding of things, but not well suited in the beginning.
The creator of Imabi argues that you need to know everything about grammar to really know grammar. I can’t argue with that. However, I can argue that you don’t really need that to get good at a language. Most of us could speak our native language reasonably well before ever learning any grammar. And grammar experts in your native language aren’t as a rule the best speakers of that language. There is far more involved in being good at a language than having a command of rules. Therefore, I think taking a shallow approach is generally better. You review the grammar rules as a whole periodically. But you actually learn to use the grammar the same way you did in your native language - by your brain naturally picking up the patterns until things start to sound right or wrong. You can refine, clarify, and reinforce through grammar books as you go.
So, yeah, I should proly just get Genki and call it a day. 