I think the problem is visual systems make it harder to create your own abstractions. You can't have all the complexity exposed in a single level, you need to be able to hide details at a lower level.

For example, each module in an audio program you're wiring together is basically a function with a ton of internal state. But you also need to be able to create your own modules composed of other modules, and maybe some lower level functions, e.g. EQ, signal sources, delay/reverb, etc.

The only visual system I've used is GNU Radio, which requires XML/Python/C++ to create your own blocks AFAICT.