I have had two separate issues:
I work for a museum and we use X10 in two different rooms. In one room, there are track lights, and there are several antique fans connected to the track lights with outlets, and there is a module on each one. The controller is also connected to the track lights. When we switched some of the bulbs to LED, the modules stopped responding. They plan to convert all the track lights to LED (Over 100 bulbs) and I will have no control over what bulbs they buy.
I use it in my home. I only use it for lighting. Up until now I only used it for my bedroom lighting and for my Christmas decorations. I recently decided to add it on some accent lighting on my living room and dining room. There are two light modules in the bedroom with dimmable LEDs plugged into them. There are two appliance modules in the living room with fluorescent lights plugged into them. There is one light module in the dining room with dimmable LEDs plugged into it. There are two mini controllers in the bedroom and one in the living room. The living room controller does not control the bedroom or the dining room. The bedroom controllers do not control the living room or the dining room. The only things on these circuits are lights (now all LED plus the fluorescent in the living room), ceiling fans, and televisions.
I havent actually tried a coupling capacitor at home, what I have done is troubleshooted with the electric stove and dryer on. If I unplug all the LED lights on the living room circuit (NOT controlled by X10) I can sometimes get the dining room lights to respond to the living room control. My temporary solution has been to run an extension cord from the living room to the dining room.