I like to sort the library (German) by “known words”.
If I apply no filters, the top lessons that show up have 10 unknown words.
However, I stumbled upon some of Vera’s lessons, Beginner II level, which I haven’t taken yet. They have only 4 unknown words for me, but they only show up if I use the Beginner II filter.
I don’t know if this is an important problem, but I thought I’d mention it. (I almost missed some of the lessons in my favorite series!)