I’m late to this discussion.
Yeah, running Ethernet in a duct is probably not a great idea.
My biggest issue is that when you have long runs of Ethernet they can inductively pick up a lightning surge. I recommend the use of medical grade (UL 60-601) Ethernet isolators at the far end of the Ethernet. The Everrstar is the cheapest I know of, followed by TripLite at over $200.