What are some alternatives?
When comparing Pianu and Flowkey, you can also consider the following products
Synthesia - A fun way to learn how to play the piano.
Simply Piano - Fast and fun way to learn piano
Skoove - Babbel for learning the piano
Playground Sessions - Online piano learning game.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Yousician - An interactive guitar-learning app.