Just an observation. The change in difficulty over the entire Japanese course feels uneven. Somewhere in the later third to quarter, it feels like the exercises and sentence structures get a lot more difficult without adequate preparation. It's possible this would be helped if there were more and better grammar notes for all the exercises. It would also be nice if grammar notes were available directly in the mobile app (yes, you can go to the website in a mobile browser but that is a bunch of extra steps when they should be supported in the application itself).
My complaint is that it makes me review very easy lessons when I have already passed the much harder lessons. Once I have finished the tree with almost no mistakes, it seems silly to ask me to review introductions or the pronunciation of hiragana.