French exercises are much more complex than Italian, French or Portuguese and give less credit
I subscribe to all four languages. I love Duolingo and have a 195 day streak. My only frustration is that French lessons tend to be more complex than the other languages at similar levels. Also, I prefer timed exercises and the French section appears to give less time credit than the others for a correct answer (and have more complex phrases), making it frustratingly difficult to successfully complete the lesson.
French from English has more advanced material than Spanish from English. There are more long, complex and idiomatic sentences, which is especially noticeable when we have to translate them from English. It also seems that they continuously inject new material into the French course -- sentences that I've never seen before, two years after having finished the tree. This difference is reflected in higher possible fluency scores in French.