I realize that this isn't what you're asking, but it's important enough that I decided to write it anyway:
If security is important to you, then you should, as a rule, assume that ALL networks are insecure. (because in the end, they all are)
Many of the more expensive network security disasters in IT have come from the assumption that "behind the firewall" everything is safe. Then when an external influence leaks in, as always eventually happens, it runs rampant and unchecked. I've witnessed this happen several times working for companies that that should have known better, like IBM.
Instead of relying on a secure perimeter, each node should be its own island of security with proper authentication and permission checks happening at each step along the way. Your system should be as secure in its home environment as it is out on the open Internet. Obviously you don't want to expose your systems to more risk than you have to, but that also means that you don't let your guard down behind the firewall.
Also, to answer your real question: yes, these systems are inherently insecure, especially in an apartment. Generally the signal can't reliably get past your power meter because the equipment installed there isn't friendly to that kind of signal. But that's usually expressed more as a "don't expect it to work when you want it to" sort of warning rather than an assurance of security. Field reports from users frequently suggest that they can see other people's network if they're physically close enough.