My experience has been both up and down with ethernet and optical, I agree, depends on implementation. At this point I prefer optical post server, ethernet pre server. I did try generic FMC prior to server, heard presentation similar to audphile1, also heard similar presentation adding audiophile switch prior to server. At this point prefer my router powered with lps direct to server vs. either of those two solutions. Network Acoustics Muon filter will be my attempt at lowering noise pre server. Post server I prefer optical via generic FMC,( soon to be Sonore OpticalModule), and Sonore OpticalRendu. This streamer much preferred to prior SOTM SMS 200Neo.
I think optical gets criticized mostly because of these generic FMC, they have pretty poor clocking and noisy power supplies. Not to say they're not helpful, they are in many cases, but one can do better with optimized optical.