The confusion appears you are only thinking about surge between devices. That is the most unlikely source. The most common is it comes in via the power and unless you are running everything on separate solar systems they are all connected to the same electrical grid. The surges that you get via ethernet are pretty much only cuased by lightning and that could cause a surge in anything that is connected including even a dedicated battery system.
The key thing to protect is the power. Ethernet cables that you actually worry about outside you would use shielded cable but you would also have to install grounds on each end to connect the shield to and even then lightning would likely get past that in some cases.
If you get a direct hit by lightening, you are pretty much out of luck. What happens then is unpredictable. The more isolation, however, the better. Of course, I'm protecting the modem electrically as much as possible from power. I can't protect everything else in the same way, except via GFCI breakers. The MOFI modem is more vulnerable, and by far, if I've got it wired to 100 other devices across 2 acres. I did have a flakey TP link router that I bought new. One plug in a switch downstream (that would be isolated via the bridge) and it instantly knocked the CMOS in the modem silly. Had nothing to do with lightening, so your most "unlkely" source was what I have already experienced. I thought here I am, having to deal with a messy return, with no internet for days. Fortunately, it reset itself. but I fiddled with it for a day, reset it many times, etc. For 2 cheap used routers for about $60, I can stop that sort of thing from happening again. Any short in a device, that would surely be no problem. In addition, it eliminates any problems due to voltage quality variations across my property. I did also buy a cheaper modem, not as fast, in the event the modem goes down again.