I learned this the hard way... Do a search (pull down on forum's top tool bar ) on PEX and you will get a lot of information. The threads will also point you to additional issues with this and other types of pex.
The short version - There have been serious issues with fittings on the oxygen barrier pex. The aluminum layer in pex is to stop oxygen from entering the water and corroding cast iron boilers and pumps. The problem with this type of pex is usually when using the wrong fittings (and sometimes even with the proper fittings) which contact the aluminum and cause corrosion. Since there is not antifreeze containing corrosion inhibiters in potable water, the corrosion can cause failure of the joint.
I am not an expert, I am just recounting some of what I have read from the professionals.