- There were several factual errors in their material and important points entirely omitted
- Japanese grammar is very flexible and their tools aren't, so they often mark you as wrong when you were correct
- Their approach to learning Kanji (borrowed Chinese characters) is completely ineffective
These flaws make me wonder if they even had actual Japanese users working on developing the tools for learning Japanese. I can't speak to any other languages, but my experience with Japanese leads me to belive they don't have expertise in many of the languages they offer, instead trying to cram their prepackaged square-shaped tools into a variety of circle-shaped holes. All languages are different and each requires a different approach.
On top of that, however, is that they don't go nearly far enough. In my opinion there's no magic bullet that can teach you a language. Each student learns differently and will need to leverage a large variety of tools. Even if Duolingo's course were good, it would only make up a small facet of an effective study strategy. Vocabulary drilling with flash cards, focused grammatical study, reading genuine material in the language, writing novel material in the target language, and regular conversation practice with native speakers are all incredibly important and distinct topics of study. Duolingo tries to mix a diet version of each into an scatterbrained, ineffectual method of study.