Good post! I couldn't agree more, I wouldn't recommend doulingo to anyone but the most basic of beginners, or someone who is going to a new country for a week or two. My biggest problem with doulingo and apps like it, is that they sell them selves on being a fast and easy way to learn a language. The hard truth is that there is no fast way to learn any language.
Many languages take years to become fluent, the only way I've found to speed up the process is to work hard everyday and immerse yourself in that language (live in a place where it is the native language if possible)
RE: Why Using Duolingo is the Worst Way to Learn a Language