Every synth app I tried put a piano keyboard at the bottom of the screen. On a phone that gets you about an octave of keys too narrow to play, which is a picture of an instrument rather than an actual one.
LUMA has a wheel instead. You hold a chord and move around the wheel to change what kind of chord it is. Major, minor, suspended, ninth. One finger, and the thing you’re picking between is the sound rather than the note names.
That cost something. There’s no way to play a melody in LUMA and people ask for one all the time. The answer is that a phone screen has room for one idea done properly, and the idea here is chords.
It changed who it’s for, too. Not people who already play, because they’ve got keyboards. People who can hear the chord they want and have no way to find it. Turns out that’s most of the people who write to me.