What are some alternatives?
When comparing Flowkey and Simply Piano, you can also consider the following products
Synthesia - A fun way to learn how to play the piano.
Pianu - Learn to play the piano in your browser.
Yousician - An interactive guitar-learning app.
Skoove - Babbel for learning the piano
Playground Sessions - Online piano learning game.
Touch Pianist - Play piano like a pro without playing piano.