Hi
i'm using the AFX3 with SPDIF connection to my moru 828es and I found the SPDIF a really robust way to connect to an interface (sold my previous one to be able to connect SPDIF).
You have the exact same sound as the USB connection with AFX3 direct to computer, but you can do many other things with the interface, depending on your needs.
That being said, I used the AFX3 as my unique interface too, and it worked great too with reaper, vsti, I7 8700k,, about less than 4 ms RTL, so no latency issue for me.
Having the ability to hook the AFX3 to an external interface with SPDIF (or AES), allows you to have a unity gain (0 dbfs) from the AFX3 (roughly the same as having the level OUT 1 knob to 3 O'clock, but with the flexibility to manage the volume level for monitoring or headphones.