UPB sounds to be very solid, but there seems to be some noise issues. Z Wave sounds promising, even Leviton is on board, however, it is based on RF which can be unreliable.
Do you mean that RF devices have an inherent reliability problem? Reliability has to do with the construction of the device.
If you mean that RF might suffer from interference from noise in the medium (air), yes your are right. Other very strong signals propagating in the area might affect it. The same happens with powerline. Noise signals propagating in the medium (wires) can cause interferece. We are talking exactly about the same type of problem here. None of the mediums are inmune to interference.
There are some differences about both mediums, though. Metal wires are a better propagation medium than air. Therefore most powerline technologies use very small signals (with the exception of UPB - but it is not perfect neither). Small signals can travel a long way - including signals from interfering devices and noise signals from out of your house. Noise is not attenuated easily (unless you install a bandpass filter, but that suppresse your signals too). In addition, the proliferation of power electronics (switching power suplies, etc.) is introducing a lot of noise to the powerline - and this will become worst in the future. Have your heard of powerline internet broadband?
On the other hand in RF the signals are attenuated a little better. Your neighbor's keyfob should not be strong enough for blocking your phone conversation (or vice versa). Digital packet communications allows me to use my 2.4Ghz phone at the same time that I browse internet using 2.4 WiFi (might be some degradation, but it is not noticeable). But there is a LOT of new RF devices added to our houses. Since UHF signals (>300Mhz) use line of sight propagation (it bounces a lot and its not exactly line of sight for signals within your house, but thats the idea) the interference from signals far away is very limited.
At the end the message is that both technologies are vulnerable to interference. None of them is inmune to it. I would say that it has to be evaluated case by case. This also has to do with your background. Many X-10 fans are influenced by the legacy technology and use to prefer powerline. I have a Ham Radio background and using RF I feel like a fish in the water. However I know that RF does not pass thru metal surfaces and all my electrical boxes are metallic. Also, my house is 100% concrete and RF does not propagate easily there neither. These are my restrictions and under these specific conditions I'm inclined to try powerline first.