The problem is that once you get past the very initial stage, you want to read ahead of what you play at the moment. At that moment Synthesia-style falling blocks notation becomes difficult to read because the blocks don't have any position reference besides the keys and no time values. Even the scrolling sheet music or tab is distracting compared stationary black and white. With guitar there's the additional problem that you can't see both hands at the same time, and usually you won't be looking at your hands at all. And if you ever want to play without help from the app, you need to learn to find the notes anyway.

This is by the way a great explanation of all the attempts to make music easier to read and how they fail: https://www.youtube.com/watch?v=Eq3bUFgEcb4