X10 is lacking in many ways, and I still use some of it, but the biggest problem I've had was with durability. Many of the components were horribly made and just broke. Stanley, GE, Radio Shack, didn't matter. Old Sears parts lasted longer but still just died.
I can't comment on how vulnerable the others are, but X10 is very vulnerableâ"from outside sockets and other units on the same transformer. North Koreans aren't going to hack your house from Pyongyang, but sneaky neighborhood kids can. I think you can filter the signals from the incoming power lines, but that's not commonly done. Also, you need to bridge the two incoming 110V legs so that X10 signals get across, otherwise parts of your house might not talk to other parts.
Still, to play around a bit, there's no cheaper way than X10.