in terms of quality, it is usually decent & accurate enough, but due to the USB-C output, depending on the type of Digital to Analog conversion (DAC) involved, there is usually some latency. ie. there'll be some form of delay in audio between what you see on screen, vs listening from the USB-C output. The lag or latency wont be significant, perhaps 2-5frames off, but then again, if there's also latency on the lcd screen or HDMI output, then it might just be compensated accordingly.
This is just my guess based on the technology involved.
I have no hands-on experience with XT30 or USB-C audio on cameras, but have used quite a fair bit of USB based DAC for audio monitoring.