The biggest problem I had with Duolingo was knowing what to do next. I much prefer a simple "next lesson" button, which both introduces new material and reviews stuff you've covered previously. But with Duolingo there's a set of available lessons you can choose, and you can either work on going down the list and opening up new lessons, or re-doing ones you've already done and leveling up those particular ones. Then there's also a "barbell" which you can press and takes you into some lesson, but I never quite figured out what it did. But I spent quite a while getting a bunch of "crowns" on the first few lessons (essentially, doing them over and over) before realizing I should be doing some forward progress as well.
My wife is a fan of the app and recently paid for premium, but I'm personally a little happier (at least for my use case of Spanish) with Babbel, which I pay for. It has a single way forward, so you never have to choose what to work on next, and that single way will bring back earlier stuff just to refresh your memory for you. It also has little grammar asides, which I found very helpful, and which Duolingo lacks.