I had taken my target language of Ukrainian, but the course was too short, and after completing it in about three months, it was just the daily grind to go over the same short content. For some reason, the course removed some content that included holiday information and greetings (not sure why), and it was outright missing key lessons on how to tell time or how to say the date, among other things. I came to find out later that for an English-speaker, saying the date is a bit tricky to do because the lessons on how to say cardinal numbers was glossed over.
It was then that I realized that Duolingo is a good introductory way to learn a lesson, but is definitely not meant to be the only way. From there, other language learning techniques such as taking classes with real people in real time, using alternate sources to learn, and incorporating daily tips and tricks to keep the language fresh in your mind are still needed.