Words not taught in sessions
I'm studying Turkish, I finished the Number session and I tried to strengthen it. But I found there words that I was supposed to know but I wasn't taught in lessons: four words in a single lesson ("binlerce", "çoğunluk", "toplam", "yüzde" in lesson 4). I had already found the same problem in other previous lessons, but it was only a word, so I didn't care too much.
Lessons usually have 6 - 10 words while for each lesson we get 17 exercises (not sure if this is changed now for some). And for a word, we often get multiple exercises like listening, translating from target to base, translating base to target, etc. Thus, usually just completing the lesson once will not expose you to all words, or even if all words do come up in some lessons, definitely not all sentences. Repeating a lesson (redo) a few times will throw up more of these sentences. Same goes for practice sessions.
I think the testing of adaptive lessons has exacerbated this issue, seeing as some people report being asked as little as seven questions per lesson. I'd definitely recommend some repetition for anyone who wants to experience the full vocabulary of a course!
A lesson can contain nine words at the most.