Only solved in the sense that you can buy additional hardware for ground loop isolation for USB and audio connections. Ground loops and RF interference are a persistent and recurring problem in pretty much all USB audio setups, except perhaps when you're solely using relatively high-end hardware that has been designed specifically for audio (cables included). Without isolation, all it takes is one bad component to ruin the whole setup.

For even a simple USB audio interface setup connected to a single audio source I would recommend isolation and balanced audio cables.

It's a "solved problem" in the sense that it doesn't matter for the use cases USB was designed for. Most people are using USB to connect a few devices at most and over short distances, so these effects are negledgible. But that is not true for stage audio.

Not even remotely.