Currently, I'm taking on Japanese. Becoming fluent with this app (or any other single app) is going to be just impossible. In isolation it will be a haphazard collection of phrases picked up at a glacial pace.
The pluses to Duo are huge, though. What some see as repetition to gain master on a mini-subject are of huge benefit compared just seeing the one or two sample sentences in a book, or hearing the same. I have to build sentences with the correct structure and I'm immediately graded and shown the answer.
As a self learner, there was nothing like this thirty years ago. With Japanese, there are particles which mark the different parts of a sentence. Sure, these are all in grammar books, but having them drilled into me and forcing me to construct sentences with them is hugely helpful.
There are lots of complaints that the Japanese course in particular is bad or leads to stilted speech, but at the structural level it seems to agree with the grammar guides and textbooks I'm also using.
Learning Arabic, I had a stack of textbooks three feet high and dozens of cassette tapes. Surely Duolingo replaces a large chunk of these, especially the starter ones.
It isn't good at bulk/breadth, and there is no speaking component.
Edit: I guess I'm saying I'm a huge fan of Duo (and other apps) as an addition to the tools available for learning a language.