While UPB is much less prone to line noise and signal sucking issues than X10 or Insteon devices there are a couple of things that I have noticed can cause serious problems. There do not appear to be any compatibility issues between X10 and UPB, I can send signals on both interfaces simultaneously and have both units respond without issue. There may be some problems with certain X10 devices and UPB devices however.
Though the UPB signal is much stronger than X10 and many people report success without a crossover to carry signals from one leg of power to the other, I was not able to reliably use my system until the installation of the crossover. You should plan to purchase and install a crossover, they are similar in price to an X10 crossover and easy to install. If there are chronic noise problems in the house you may want to talk to the UPB folks about an inverting vs a non-inverting crossover as one may be able to invert and cancel out noise coming from one leg onto the other.
The first way I managed to completely halt my UPB transmissions in the house was with a non-dimmable CFL bulb that I had placed on a dimmer. This isn't usually a problem as long as the dimmer is set in XTension to non-dimmable so that only ons and offs are sent, but in my case I had forgotten to change the setting in the database and so the CFL in question had gotten dimmed to 0 instead of turned off. This was enough to somehow fill the powerline with enough noise that no UPB communication was possible in the house. Turn it all the way off or brighten it a bit and the noise would go away. So make sure that non-dimmable CFL bulbs are actually turned off and not just dimmed to 0. I believe that this same problem might occur with non-X10 dimmers, manual or possibly even other UPB dimmers incorrectly dimming a non-dimmable device.
Jeff Volp writes an excellent essay here on X10 reliability. He talks about some problems being beat frequencies of several smaller noise sources that can add together to create beats that are large enough to cause problems. I believe this is even more common with UPB noise in a system that is shared with X10 dimmers or possibly other dimmers. I have many dimmable lamps set to “simulate preset dim” in XTension. They have worked great, but for some reason when certain combinations of them are dimmed to zero the noise level as measured by the UPB interface can become so high that it stops all UPB communication. In my house there are 3 highly noisy simulate preset dim dimmers. One seems to be the instigator, if it's dimmed to zero then I cannot dim either of the others to 0 without stopping all UPB. As long as that one is turned off fully then the others will operate without causing any trouble. I have other lamps that are also dimmed to 0 that do not cause any problems, I don't know why they are different. This was very difficult to track down due to the necessity of having more than one unit dimmed to 0 at a time. If you're experiencing high UPB noise levels start by brightening any lamps you have dimmed all the way to 0 and you may find the problem. To be clear, these are regular X10 wall swiches with regular bulbs in them. NO CFL's no fancy switches, nothing.
If the problem is simply one of propagation then bad or loose or old wiring seems to be as much a problem for UPB as it is for X10. Specifically I was attempting to get signal to a desk lamp and nowhere in my bedroom that I plugged it in would it work. But elsewhere in the house it was fine. My bedroom is not one where I have updated the old backstabbed outlets that came with the house to higher quality screw terminal devices. At one point I pushed on the plug in an outlet upstream of the lamp and it started working. As long as I pushed on anything plugged in there it worked, if I let the plug alone it would block UPB at that outlet and keep it from getting downstream to other outlets. I've since replaced those outlets. There were no visible problems with them, but the connections weren't perfectly tight and they were old and a little tarnished and such. Upon replacing with a new outlet with pigtailed connections to the screw terminals the problem has gone away.