As of this writing (October 2020), the Japanese language part of the app is quite new so there will be some bugs to iron out. What I have seen so far:
- on the kana training many picture cards will call for katakana when the answer is in hiragana. This should be easily solved.
- at least one pronunciation does not correspond to the furigana; in 雪が降る, the 降 is pronounced 'fu' but the furigana indicates 'o'. I needed to consult jisho.org to check the discrepancy. If there are a lot of these, this could cause a problem.
The app and perhaps the methodology will need to take into consideration context.
- For example, the language in Genki is a bit less formal than the language used in the first set of cards of Fluent Forever App. The example that comes to mind is FF's 'yūshoku' instead of Genki's 'bangohan' (both meaning supper). I don't really have a problem with this, since more formal at first is safer (especially for say, an adult tourist) but the beginning student might not know this.
- I'm a bit worried about the photos available for 'usui' (thin, adjective). Photos showing a thin book (correct) are good, but the rather thin young ladies might not be. I believe the thin young lady should be graced with the polite te-verb 'yaseteimasu' instead of the adjective. No doubt the translators are correct, I am just learning the language (and have been for some time by other means), but the photos available might not be.
My take is that I enjoy the FF app and the methodology seems sound (although there is no teacher/student dialog yet). However, it should be just part of a student's toolkit for now rather than an all-in-one solution.