Are powerline ethernet adapters inherently secure?

You do get some security from the way your fuse box is connected to the mains.

In principle you should get a good signal across any part of the wiring in your house that is on the same phase, and you shouldn't get any on the other phases.

In reality though, that isn't quite true - depending on your fuse box, you may get some bleed over onto the other phases, and you will almost definitely get leakage outside your 4 walls.

This is why encryption was put in place on these types of things - a neighbour may be able to sniff your traffic.

Things that help with security, because they hinder signal strength - surge protectors, UPS's etc. but those don't prevent an attacker.

tl;dr

Encrypt, because you will be leaking signal. Not so much wirelessly (it can be done but it is tricky) but just across the existing mains wiring.

I live in a detached family home in Texas. I have a pair of Trendnet TPL-303E powerline adapters and have experienced the signal bleed from my next door neighbor. I ran the Powerline utility that came with the adapters and could identify two other powerline adapters using the same network name. I got anywhere between 10 to 20Mbps of throughput between their adapters and mine. I even seperated my powerline adapters from my router and connected them direct to my pc to see how bad the bleed was. I was able to access their router, watch streaming video and see the computers on the network. I also noticed they had gotten IPs on my router also. I've since enabled security.

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.