Not all fmc created equal, I've had cheap generic, now use Sonore OpticalRendu which also serves streamer function, meaning Roon Endpoint. This separate from a much more expensive custom build streamer which now only does Roon Core duties. While this makes for more complex streaming chain, many have found two streamer setup preferable for Roon. I highly doubt on board fmc within routers or switches provides sound quality on par with dedicated high quality fmc. I'm also skeptical as to quality of stock fmc within streamers. SFP modules, also called transceivers are variable quality as well, I replaced stock with Finisar 1475 to good effect. Finally, quality of AC to these devices important as well, meaning quality lps, DC cables, power cable and power conditioning to the lps. I've optimized every single component in streaming chain, it all matters.
Aside the above, quality I2S implementation and connection to dac superior to usb input, this includes dacs with very nice usb boards. Replacing dac internal master clock with external clock can be nice upgrade.