I find it to be a great app, especially for beginning to acquire vocabulary!! It also has a lot of flexibility, and the app being open source is awesome! I don’t use it as much for my German anymore, because I admittedly find it a bit boring. Ultimately, it’s a GREAT ressource for beginners and smaller languages, but for the bigger languages at intermediate/high level, I personally recommend focusing on comprehensible imput.
There’s an app that plans to be a free, community-based clone of Duolingo, with support for all languages as long as there are skilled contributors, but it’s still in development. The app’s name is Lingonaut, and the website is https://lingonaut.app/