It's not a stupid question. X-10 dies pretty quickly and couplers may be required to couple the X-10 signals across split-phases
UPB has passwords that are assigned to the network, thus your neighbor bay receive the signal, but you can't activate his/her stuff. When commissioning the devices, you have a certain amount of time to commission it similar to pairing a bluetooth device.
I believe Insteon is not strictly a powerline protocol. It uses wireless too.
X-10 is not the way to go for a new system.
UPB has some interesting modules.