@WilJo 

Hi,

Where I live it is very usual that the wall sockets of different rooms have separate circuit breakers, which means the Powerline signal passes through two circuit breakers on its way from one room to another room.
A third circuit breaker would certainly add a bit more signal losses, but I don't see it being a significant problem.

But nonetheless, predicting the performance of Powerline adapters in advance is pretty much impossible. Therefore, buying from a retailer who has a good return policy can be worthwhile. 

By the way, the phrase "won't work by design" is more commonly used when it comes to wall sockets being wired to different phases of a 3-phase electric supply. Like, if a building is connected to 3-phase power, then different floors or section of the building are usually wired to different phases, for the purpose of balancing the load between the three phases. This is a big obstacle for Powerline adapters.