As noted by @Kartman, EtherCAT at the physical layer is effectively 100MBit ethernet. I have had extensive experience plumbing 100MBit ethernet through robotic systems that operate unsealed in some really nasty environments and it's pretty reliable:
-we would use JST PA, SH and GH connectors, crimped by hand
-we would use 24awg standed wire twisted with a hand drill
Certainly not ideal!
Our ethernet links would work well until:
-multiple shorts to chassis occurred (single point was kind of okay)
-corrosion caused the connection resistance to increase
with regards to resistance: My general guideline was if a fully connected link measured less than 2 ohms across a pair on a multi-meter (usually 4 to 4.5 ohms indicated with lead resistance factored in) then it was good to go. Above that I would service the wiring and connectors.
As for diagnostics, if you can connect a linux running device direcly to the link in question,
netstat -i will report the number of dropped packets:
And with two linux appliances
iperf can be used to benchmark the link: