Insteon is somewhat less likely to be bothered by noise because of the tendency of all the repeaters to saturate the "network" with the Insteon signal. It's also less likely to be bothered by signal sinks for the same reason. Plus, it probably has a far narrower band of sensitivity to noise. Still, it uses 131.65kHz while X-10 uses 120kHz so it will be subject to some of the same gremlins as X-10. I noted that in my initial review a few months back. OTOH, it also has 10mV sensitivity.
Not all PLC systems will have the same sensitivity to noise. The HomePlug system can adapt to noise, shifting to other frequencies to avoid interference.
Your ESM1 will also provide a measure of Insteon signal level. It might be useful to others if you would post the results of measurements from various points in your extensive Insteon "network".
I'm hoping to send Bruce Robin a ZX-24 based version of roZetta by the end of the month. Among the things it will do is allow people to use X-10 RF to trigger Insteon actions.