its possible to get rPI3 latency down to acceptable levels, once you use an external audio interface... which this must be using (if not, its going to be terrible, so I assume they are)
I'm confused why the audio interface they went with didn't included an audio input, seems like an obvious omission.
hopefully they will also be using a buildroot distro to ensure it boots up 'instantly'.
I think this is a nice project, a good example of combining available technologies to form something really nice.
you could build this with a rPI3 + 7" lcd + PiSound + Arduino/stm (for analog io) + sliders/knobs, but it would take a faIr amount of effort to then build the associated software, and make it work nicely together.
perhaps this says something about the state of rPI software, with a plethora of touchscreens available for it, it would be possible to build something similar to the various ipad options, but I don't think ive seen anything approaching the quality of the gr-1 or ipad on the rPI.