Emphasis on Abjad
I took three years of Arabic in college and wanted to try out the Duolingo course because there were a lot of holes in the pedagogy and I haven't practiced Arabic in over a year anyhow, so my experience is going to be a lot different from other people.
I found that I was able to test out of all the different sections, but not necessarily because I already know all the vocabulary. It seems like half of the questions I was asked were nonsense words that are supposed to reinforce the abjad. We honestly spent a relatively short time learning the abjad, it's something that people assume is the most difficult part of the language but in my experience is actually one of the easiest. There are a lot of benefits to setting up a system where people get slowly acclimated to the writing and phonology system of the language but I'm on the final lessons and I still have to start off every section with twenty questions teaching me how to identify letter clusters.
The set up is not very helpful for me because I have a lot of exposure to Arabic writing and my biggest issues are limited vocabulary and speaking/listening disfluency. A lot of my friends write in Franco-Arab and while the system used in the course is a lot-lot closer to that than most of the teaching resources I've seen there are still some differences and I don't know if that skill is developing either since the letter clusters are not connected to any actual words or phrases. I think it would be helpful to keep the system but replace the mostly nonsense words with at least some words that students will learn in the actual lesson. This would have the effect of foreshadowing the vocabulary, creating associations between the abjad and Franco-Arab, and reducing frustration for experienced learners who want to solidify their skills/relearn the language.
Hoping this did not come off as too critical, like I said I already like the style of this course more than the way we were taught in school.
For me as a native speaker, and noticing some posts from learners in this forum, I can say that this course needs a lot to be achieved still. Besides, it is supposed to be MSA, yet I do find some dialectical words were used, needless to say, grammatical errors. The text-to-speech machine used here is not helpful, and they are better with a real voicing from a native.
I found the writing easy, but i think that was partly because the lessons were so thorough, adding a few more real words would be useful though.
For Arabic, the courses on Memrise seen to go a lot further on vocab. https://www.memrise.com/course/1325843/arabic-7/
They kind of skim over the letters though, i'm finding doing both seems to work well for me.