Duolingo Hack
I used Duolingo to learn Japanese for a year after which, Duolingo replaced the Heart system with an Energy System (By far the worst change, which everybody hates). After this I stopped using Duolingo. A few days ago, I found that if you use Duolingo on PC, you still have hearts, so I guess that's a W.
For those who don't know: Duolingo switched from Hearts to Energy. Before you used to be able to do as much lessons as you wanted until you made 5 mistakes and you ran out of lives. Even if you did, you could replenish them. Now, even if you made 0 mistakes, the game forces you to wait for your energy to replenish as it decreases with every question.
Hope this helps. If I'm wrong about anything, please let me know.
I still wonder how Duolingo could make something worse than hearts .