I don't think the X10 protocol is an issue, but one definitely has to do more than install the modules for things to work well. Once an XTB-IIR is installed and filters used to eliminate noise/signal suckers (located via XTBM), PLC should work great.
Prior to investing in modifications, I was very close to throwing in the towel. Adding the XTB-IIR and filters fixed the PLC issues. Adding an antenna mod to the CM15A, adding RF amplifiers, and centrally locating it on the booster port of the XTB-IIR fixed much of the rest. I'm still not happy about the range of the motion sensors nor the "weather proofing", but all of the PLC components I've used seem more than acceptable.
So, I suppose the only other thing that I'd say Insteon has over X10 is that every module is a repeater (as I understand). If you have range issues with X10, you have to find a signal booster which is much like unobtainium. I know jvde makes one, but I haven't seen on offered by X10 before.