> The second thing is that domestic buildings usually do not come with a consistent ground plane.
Ordinary unshielded copper Ethernet doesn’t care: it’s transformer-isolated at both ends. Shielded cable may object to carrying any substantial amount of current through the shield.
Anyway, there are a handful of good reasons to use fiber:
- Length. Copper is specced for 100m. Panduit will sell fancy copper cable that they pinky-swear works for Ethernet at 150m. Single mode fiber will work at silly long distances.
- High bandwidth. Copper will do 10Gbps. High speed specs exist, but there is approximately zero commercial availability of anything beyond 10G using copper at any appreciable distance. Fiber has no such problems.
IMO if you are running fiber anywhere that makes it awkward to replace (i.e. not just within a single room), use single mode. Multimode fiber has gone through ~5 revisions over the last few decades, and the earlier ones have very unimpressive bandwidth capabilities at any reasonable distance. Even the latest version, where truly heroic engineering has gone into reducing modal dispersion, relies on fancy multistrand cables for the faster Ethernet speeds. Single-mode fiber, meanwhile, continues to work very well and supports truly huge bandwidth at rather long distances, and even decades-old fiber supports the latest standards. And the transceivers for single-mode fiber are no longer much more expensive than multimode transceivers.