I tried this out for the past hour or so. It takes the 9 most common letters in English and makes them easily accessible through one touch down, whereas all the other letters require a touch+swipe.

Multi-lingual support is problematic, the 9 most common letters differ per language, even if you stick to those based on a latin script. So either you end up learning multiple layouts for multiple languages, or end up doing way more touchswipes than should strictly be necessary.

Granted, I am a pretty fast and accurate two-thumb typer on a qwerty keyboard (AnySoftKeyboard) on my phone, so it might not be for me. Perhaps someone who struggles more with accurately hitting individuals keys would benefit more from this simplified keyboard