I love that Duo streak is hard to lose, unlike other apps where if you miss a day streak is gone. The goal of app is to help you keep learning. In life things are bound to happen because of which you might miss couple of days or more, it make no sense to penalize you for that. I have been trying to maintain streak in other apps but I when I miss a day, I am not motivated enough to go back and start again. Also, streak is only one gamification point in Duo, it's ok if it isn't perfect. Leagues, stories, medals provide enough motivation to keep making progress. I have tried to learn french multiple times and nothing made me stick to my learning routine like Duo has. It might not teach the language to hold a conversation, but I like how much progress I have made reading front.