As for Duolingo, I have found it to be a critical but small part of learning from scratch. In general my learning process is:
Exposure > Produce basic sentences > Grammar > Vocab > Conversation
Duolingo is for steps 1 and 2 only, but it's SO important because that's the only way you can actually start learning. Dumb repetitive exposure is necessary, especially if it's a different script. It's also really boring, so the addictive gamification is necessary.
I used it for my self-driven from-scratch language (Japanese) and it was the only thing that worked for me after bouncing off many times. Overall I spent ~300 hours learning it over the course of 9 months, and only about 20 hours was Duolingo, but it was still the most important step. It went kinda like this:
Try studying character flashcards > bounce off for a few months > try character handwriting apps > bounce off > try vocab > bounce off > try Duolingo, which actually created a habit of exposure > finally did flashcards to memorize the characters with less exposure > after about 20 hours of Duolingo, switched to Youtube and blog posts for grammar learning > after about 100 hours of grammar, moved to HelloTalk for native conversation > added Memrise for top-500 words vocab training > continued regularly with grammar + HelloTalk + Memrise until the trip > dropped it all after the trip and got super rusty
_foolmo_
he says listen to my story this maybe are last chance