I forgot to mention that there have been some reports that a Z wave wall switch, in wall relay, or in wall receptacle may create interference on a circuit which can interrupt a powerline transmission on that same circuit. It doesn’t seem to be a problem just having zwave in the house. As long as they are on two separate circuits you’re OK. But if they’re on the same circuit it can be a problem and it can be a problem that is hard to diagnose unless you know to look for it.
One other thing that you may already know and it may even be related: powerline is famous for creating static on speakers that are plugged into the same circuit. Again, no problem even just being in the same room as long as they are on a different circuit, but the noise from the powerline can sometimes create noise on a speaker plugged into that circuit.
So this is another of those “all home automation is local” things, what might work perfectly in one house might not work at all in another.
There is a reported problem in England that powerline can interfere with wireless signals in a band that theoretically might affect zigbee, but I’ve never heard of an actual problem with this, and I don’t think it’s been reported in the US. But of course wiring in the UK is different so again, it may be a local issue.
We have powerline in two rooms on opposite ends of the house, each connected to an Xbox, with zigbee lights in those rooms and have not had any issues for what that’s worth. But we’re in the US.