Slow writes are generally true, I suspect for similar reasons that most usb sticks suck: trade off between price, capacity, longevity, and thermals. I’ve used a 1TB sd card for years withoutany issues, but now have everything on a NAS instead, so the slot sits unused.
USB-C audio isn’t simple. Does the phone have built in DAC with usb pass through (most dont or its poor quality), or do you need usb DAC? Also, not all usb DACs will work on every device.