A Pi+Digi+ Pro is excellent when you need SPDIF (optical or coaxial).
For USB it is a different matter: if you want to follow the SBC route, an Odroid C2 or a Cubox would be better choices.
Upping the game a bit: Sonore’s MicroRendu is supposed to be very good (if not the best) at what it does. Don’t know the Node2 – it has just become RoonReady as of last week. The Auralic mini is not Roon compatible – and from what Auralic tells us, it will probably never be.