I was not much of a fan of the “crown levels” to begin with, and I still feel that way personally. But I have no reason to doubt the claim that they have been hugely motivating to a lot of students. After all, I am not a totally normal person, even among the people who learn new languages.
But there is a problem with how crown levels work now, that I believe is an objective flaw. Correcting it would probably make Duolingo better. As far as I know, there has never been an experiment to check whether this would work.
In the earlier versions, daily training would influence the status of the circles you trained. I mean: Each time you used the “dumbbell” training icon that gives 10 XP, it would select exercises from one or two skill circles. Possibly more, but mainly one or two. And when you had finished your exercises for the day, those circles would have improved. They would be completely gold or at least more gold than when you started. If you did enough exercises, you could keep the whole tree gold, or as far as you had come. I did that with Japanese, and mostly with the others while I studied them actively.
But now, when you do the general training exercises, there is no effect on the circles. No matter how many dumbbells you do, the crown numbers don’t change at all. To change them, you need to do the specific crown exercises for each skill. This makes no sense. If you translate the same sentences, it should have the same effect, because it has the same effect on your learning.
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One thing that may work right about this, I am not sure yet, is when you max out a skill circle to crown level 5. It looks to me as if you stop getting random exercises from it in your daily training, but again I am not sure. I only started maxing out the early circles once I discovered the feature to let you test out of crown levels. But I haven’t had any of those I maxed out since, so they seem to be at least more rare.
But the thing is, you should be able to max them out (or close to max, at least) by doing sufficiently many ordinary exercises. Maybe you could stop at level 4 in case there are people who specifically enjoy the experience of maxing a skill and don’t want to risk doing it by accident. But the first four levels should be doable. In fact, when crown levels were introduced, they used the stats from my past dumbbell exercises to establish the crown levels of the skills I had learned. So it is possible, they just don’t do it anymore. Or at least I have seen no hint of it. That is the biggest flaw I see right now.
(Others probably see other flaws, like having to spell English correctly when learning Japanese. But English is my third language, so I am not offended if my English is corrected by a computer.)