With PEX the water in the line will freeze....but the pipe won't burst it will expand. So you may lose your water service but when it thaws you should be okay. The claim is any certain spot on PEX will expand and contract multiple time before it bursts.
I know for a fact it will do it at least one time because a few years ago we lost power in one of our investment homes during a cold snap....and nothing happened. Took a small propane tank, one plumbers use to sweat line, ran it about 12 inches away from the pipe and kept moving it back and forth to just warm it up. You could here the ice in the line cracking and about after 15 minutes of just warming up the pipes we had water.