We've all heard of Duolingo, and we've all probably used it at least a little bit. I have friends who think it's amazing.
But what are the drawbacks to using Duolingo? In particular, have any studies been done which show particular weaknesses in the Duolingo system versus other learning methods?
This is basically a summary of what this website has to say.
You are not exposed to natural sounding conversations and sentences (at least, not until very much later as you reach more advanced lessons). In this respect, Duolingo is in dramatic opposition to other language methods such as Assimil, Teach Yourself, or Berlitz. I don’t know how often you use the words “elephant”, “lion”, “snake”, or “horse” in your daily conversations with people, but no matter how useless you think this vocabulary is, you’ll have to go through the lessons that introduce it whether you like it or not.
As stated above, because the program progresses in set stages and introduces vocabulary in what I call “boxes” (animals, food, jobs, furniture, etc.) rather than in a more natural fashion, the sentences you are exposed to, in a large number of early lessons, are essentially useless and at times nonsensical. For example, I’ve come across such sentences as: “My snake eats your cake,” “I have our cow,” “Their elephant drinks milk,” “The knife is in the boot,” “We come from the women,” and many more similar ones. Probably not the best arsenal of sentences to impress the native speakers on your next trip overseas…
Duolingo has no natural sounding conversations, the stuff you would normally find in most good textbooks. Rather, you’ll only be exposed to short phrases/sentences.
Duolingo uses a computerized voice system for all of its listening exercises, so you’re not introduced to how the language really sounds. The voice is dry, non-rhythmical, and well, it sounds like a computer. Because of this, I found the listening exercises quite useless, and you will simply not learn to speak or listen to the language correctly.
As stated above, Duolingo does not offer any explanation of grammatical structures as part of its mobile platform. However, grammatical occurrences are explained in the desktop version of Duolingo (as pointed out by @Ven in the comments).
Except for the issues already mentioned in the other answers, a big problem of Duolingo is that by grouping words in 'semantic clusters' (e.g. ANIMALS: elephant, lion, snake, horse, cow, mouse, spider etc.*) acquisition of these words is actually made more difficult, due to a phenomenon called 'interference' in psychological research.
There is a lay-man's description of 'interference' and its effect on language learning in this article on the site Fluent Forever (in a sub-section with the title "A quick note about order"), and there you will also find a link to a scholarly article about a relevant scientific study.
*This applies not just to simple nouns, but also to more grammatically relevant clusters in lessons like: PREPOSITIONS: all-, alla, alle, allo, agli etc., or POSSESSIVES: mio, mia, mie, miei, tuo, tue, tua, tuoi etc. (examples from the Duolingo Italian course).
Another issue--for me at least--is that only a small percentage of all the exercises are translation exercises into the target language and there are a lot more L2 -> L1 exercises.
[Because of this I have started the 'reverse course' as well, i.e. I am not just doing the regular English -> Italian course, but also the Italian -> English course (best to create another account for that purpose though). Of course this is not 'the perfect solution' (I'll skip enumerating the drawbacks) but it works fine for me and there is definitely quite an increase of L1 -> L2 exercises.]
I am a Duolingo user, currently working on the Spanish course. I do find it to be beneficial, but it does have its limitations. A few that I've noticed are:
As has been mentioned, the sentences used are often not very practical or even sensical; how often do I need to tell someone "I am a penguin" or "that is surely my elephant"? However, some may find that this makes the lessons interesting and memorable.
The listening exercises for many courses use a computer voice, so while you get some practice identifying words in speech, sometimes the computer pronunciations are unclear, and you also don't get the native speaker intonation.
The explanations of some vocabulary and grammar are confusingly incomplete, for example, in the Spanish course, "gustar" is translated as "to like", with conjugations "yo gusto", "nosotros gustamos", "ellos gustan", etc. So you would definitely have to go to some other source to get a better understanding.
There is an inherent limitation in relying on translation to/from another language, as Duolingo does exclusively. One speaks most fluently when one no longer is mentally translating from the native language or another language. Words in one language often do not have an exact equivalent in another, and Duolingo does take this into account to some extent by translating certain phrases as a unit. But, while translations are good for beginners, in order to be fluent you have to think directly in the target language, which is not well encouraged by Duolingo; learning through context and real-world association are better for that.
There is also limitation is using isolated words, phrases, and sentences. While knowing how to say "I do not reject food" is useful in some circumstances, it's still just an isolated sentence without context. I personally find it easier to learn words and grammatical constructions in context, such as a story, a conversation, a letter to a friend, etc. This gives more perspective on usage, pragmatics, idioms, etc.
All that being said, there are a lot of good things about Duolingo; but the question only asked for the drawbacks.
Quote from "Does Duolingo “Trump” University-Level Language Learning?" by Stephen Krashen:
Duolingo is a web-based self-paced language teaching program that guides students step-by-step through a sequence of tasks, largely based on translation. It is clearly aimed at conscious learning
Later in the same article:
Both Duolingo and most foreign language instruction are based on conscious learning, as was the test used in Vesselinov and Grego. There is a great deal of evidence showing that conscious learning does not produce true language competence. Among this evidence is the consistent finding that methods that promote subconscious language acquisition are far more effective than traditional methods on communicative tests and are slightly more effective or just as effective on tests of grammar (Krashen, 1982, 2003).
Link to the full paper: Does Duolingo “Trump” University-Level Language Learning?
So I'll answer this a little more personally.
Duolingo is useful in many ways. I used it to learn Esperanto and to refresh my Spanish.
A drawback to Duolingo is that it is, at least to me, boring. The same answers and the same questions, over and over again! Do I really have to go through all the animals? Do I really have to repeat "tomatoes", "tomates" if I haven't done any repetition in a while? I want to repeat what I really need to know, the things I miss in the repetitions, not the very first lesson!
Even if I "skip" a certain number of levels, repetition still starts at the very bottom. I'd like some kind of more flexible repetition system - like, combine the tomatoes with complicated verb forms that I just learned.
The system is rigid. If I want to know about adjectives right now, I can't do so until I have finished Animals 1. If I have a textbook, I just go straight to the chapter I need.
There is just so much to repeat that the things I really need get lost in all the potatoes, tomatoes and elephants.
What I guess this boils down to is that, compared to a text book or a teacher (or something innovative like LearnYu, where you get a number of exercises around the specific word you forgot, not just the same question or no progress on the whole category) Duolingo isn't flexible or responsive.
It's flexible and responsive to a certain extent, and I wouldn't tell people to avoid Duolingo for this reason. You just have to know what you use it for, how and when it is useful to you.
Happy language learning!
In addition to excellent @fy12 answer
Duolingo is annoyingly picky about spelling. If the course designer decided that the translation of "viro" from Esperanto is "the man", writing "man" is a failing answer. Ignoring optional parts of the answer (like "the") is relatively trivial (compared with all the other functionality), not I'm not sure why it was not implemented.
A smartphone is really bad device for typing, especially non-ASCII characters, but there is no way to disable tests requiring typing for a person who is not interested in learning typing via Duolingo on a mobile device. As a result, the learner will get the most of the typing tests which are most useless, and fails most often because of stupid typos. This is exemplification of a frustrating experience.
So basically if a person is willing to work hard, my feeling is that the time would be better spent in some system more friendly and better suited for mobile user, like Anki.
