Hi @tony-j,
I think there are not many zyhtians around using a USB-Interface - at least that is my observation.
My guess is that having a small one-box-device is the way to go for most because the whole GUI is made for being controlled by a certain case design centred around a special display and knobs layout.
You can find examples here:
Besides additional driver issues with a lot of sound cards USB seems to add to latency…
About your second question: For the most part it woks fine on the Pi3 but zynaddsunbfx is totally able to cause xruns if a very complex patch is beeing used especially in combination with high polyphony. The synth is sadly just using a single core of the Pi`s 4 cores.
Regards,
Martin