That's a bit over the top, in my opinion. I've tried plenty of courses, and Duolingo is pretty good to get a hang of the basics of a language.
I'd say, in my experience, the hardest part of learning a language is getting started, and I feel Duolingo is perfect just for that. To get deeper knowledge and become more comfortable, one should probably switch once they start feeling more comfortable with the alphabet (if there is a specific one), and with the basic vocabulary and grammar.
EDIT: Forgot to add but another advantage of Duolingo, is that it's also great to get a taste and basic feel for different languages; and that can be especially useful for someone who is looking to learn a new language but can't quite decide on one.