Someone mentioned on r/DuoLingo so I installed in order to try it. My biggest problem with it was the UI and lack of options to configure it. I would really like options to make fonts and buttons bigger. Altough I changed phone font size in order to use app, small height of buttons on the bottom are really getting on my nerves.
Spaced repetition is a highly effective method for learning languages, particularly for vocabulary/character acquisition, but it is not the only method and particular implementation of it in the software, really matters.
old HelloChinese course is still there in the app... New one in making so far looks more like DuoLingo, however, LingoDeer app is alternative that right now is better for Asian languages if you want a little bit grammar stuff while learning.