Like anything else, the downside of "PEX" plastic piping is the common point of failure which would be the fittings.

There was a large class action lawsuit filed about a particular type of fitting used for PEX.

The upside of PEX is that it is flexible and you can make "homeruns" from manifold to manifold. Therefore eliminating the need for numerous fittings and connectors that would be required to run copper.

Without a manifold to manifold type system, I would argue against PEX.

Another upside is that copper is getting more expensive all the time.