I used to connect my Gustard X20U via I²S to a Singxer SU-1 USB DDC. Any difference I thought I heard was probably down to the SU-1’s isolation and jitter performance rather than the I²S link itself. Personally, I wouldn’t recommend using I²S between external devices: the standard was never intended for that, and incorrect wiring can easily introduce issues. Coaxial or Toslink are well-defined, measure transparently, and are much less error-prone. From my perspective, I²S in consumer gear is mostly a marketing feature rather than a technical necessity.