I did an A/B comparison with the optical and COAX on the ZP90. Aside from impedance changes and volume, they sounded very very similar. But I did note a sharper and less rolled-off sound with the optical. Due to my setup I had to put the optical on the PS3 and the COAX back on the Sonos since my Cary DAC only has one of each. And of course, each rig will sound a bit different with all the variables... of course! But coming from a Sonos? It's not exactly the most demanding hi-end piece of gear out there.
I put a $200 digital COAX on there vs. an $80 optical. I'd have kept the optical if I had the option and it's not because of price. It really sounded just a touch cleaner. That's all. You would not know the difference if someone changed out your cables over night and you listened to it the following day.
And limitations? Not really. There are debates over using CAT6 vs. CAT5e and so on but another improvement I had was when I replaced all the CAT5e cables with shorter CAT6 patch cables. The bass was more pronounced and very clean and tight. I was like, wow. Who knew? Again, cables are hit and miss in about every system. But the CAT6 patch cables are cheap and readily available and I consider any addition that is overkill is the addition I have to worry about upgrading or changing last. I'll never have to wonder if they are limiting my system because as of right now, the Sonos is far from capable of putting out that kind of bandwidth.