USB has a huge problem compared to MIDI cables: it's not optoisolated. That alone almost takes it out if the running for controlling synths via laptops.
To be fair the approach taken by MIDI for optoisolation (and getting essentially differential interface for free with that) is in fact one big ugly hack.
Connecting a controller keyboard to a laptop isn't an issue because it shares power and ground with the laptop. But synths on separate power are another issue entirely. For example, the Waldorf Blofeld has a USB MIDI connection; I attach mine to my Macbook Pro and its audio presents a ground hum and an electric buzz whose pattern matches the Macbook's processor utilization. Attach via 5-pin DIN MIDI and the problem is gone.
USB wasn't designed for this use case, and it is a very, very common problem.
Except sometimes it is: I have an audio workstation with two "yellow" USB ports (as opposed to standard USB2's black and USB3's blue) that are dedicated audio USB ports with additional circuitry specifically for connections to audio devices. My Komplete Audio 6 kept buzzing and letting me hear my cpu activity in my old system (is it both amazing and the most annoying thing ever to hear your mouse move as HF signals), and has had perfect audio since switching to those for-audio USB ports on the new motherboard. As far as I know only Gigabyte makes these (my specific board is a z170x gaming 5) but they exist, and they're great.
Certainly, UBS in general can't be used for this purpose, but if there's true industry buy-in for MIDI over USB then it's not hard to imagine more and more USB hardware will get made that can compensate for that problem. For instance, I wouldn't imagine laptops to get isolated USB, but a shiny new post-MIDI-2 audio interface sure would.
Ground loops. Stages are very electromagnetically noisy, mainly because of lighting dimmers. If you connect a mains-powered computer to a mains-powered master keyboard via a USB cable, you've just created a big antenna for all of that RF noise. That noise will travel through the ground plane into your USB audio interface.
In that environment, isolation is a constant concern. Connections that are inherently isolated make fault-finding far more straightforward and obviate the need for external isolation boxes.
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.