I did Duolingo seriously for more than 370 days in a row. Spanish: I speak french and English so Spanish is relatively easy to complete. After completing the course, I tried having conversations with people (with friends but mostly uber drivers) and I was surprised how many times i learned it all wrong. I had to read a lot or children book to remedy that. French: As a french speaker, I went through it as a meta cours…
It's mediocre at teaching you how to write, bad at teaching you to listen, and useless at teaching you how to speak.
I've found that if you just grind through lessons without actually reading the supporting material, you don't learn much at all. If you actually read the supporting material you'll learn a lot more.
Duolingo is best used as a tool in a toolbox of language learning tools. It's good for reinforcement learning, but not great for actually developing an understanding.
As an analogy, you could learn algebra by looking at a lot of examples and working through worksheets of algebra problems, but you're not going to actually understand what you're doing, or learn very well.