Just when I convinced myself that UPB is the way to go ... I come across this post. It sounds waaay too much like working with X10. Say it ain't so!
In my opinion it is the way to go. I've haven't had a missed command in 2 years using UPB. There are some rules of course and depending on your exact situation fine tuning is always a good idea.
First an foremost, don't let anyone tell you that you won't need a phase coupler, you will, if not at the start, at some point. Just bite the bullet and put it in.

Depending on the size of the home you may need more than one, not usual but I've seen it happen.
Give yourself all the advantages, go for a direct line to the electrical panel for the PIM. It's not necessary but it avoids lots of the pitfalls and gets your UPN signals to the panel nice and clean.
Do not wire UPB switches live, turn off the breaker. I've seen lots of electricians wire it live and blow the chip while doing it. Not that I mind, i get lots of work fixing them and cultivate new customers in the process.
I've not had a problem yet mixing Gen 1 and Gen 2, not that it couldn't happen to someone, I've just not seen it. Personally I'd suspect a weak device before I'd think of a problem with mixing the two.