Not sure of your definition of “powerline circuit”. If you mean on the same breaker, that certainly wasn’t the case for me. They worked fine regardless of whether they were on different breakers.
I certainly understand why the signal strength would be greatly attenuated if they are on different 120V phases (about 1/2 the 120V circuits will be on opposite phases), since the transformer’s impedance would block most of the signal unless you install a capacitor between the phases to mitigate this (old X10 trick).
But, as I posted, they wouldn’t work for the long run down to the pond.
So, I bought a Ubiquiti NanoStation and located it in my Attic. And a PicoStation for down at the pump. These work great! I can cover all 8.5 acres of my property with very good signal quality and high data transfer speeds. In fact, I still got a good signal over a mile away.
Originally I did repeater mode. That worked great, but only WEP security is supported in that mode. So, now the Nano is configured as a bridged AP to my existing DSL/Wifi router.
The Nano and Pico are a bit pricey for hobby stuff, but for a home improvement cost, they don’t look so bad.