Silent blowing: Expansion tanks do not give any indication when the bladder breaks. And that is a certainty, if anyone works on well bladder tanks, in about 3 to 7 years.
Gunk: the air half of most tanks is raw steel, and condensation causes bacteria and rust to grow. When the bladder silently ruptures because the air charge was lost
[all tanks lose air on a predictable basis and are rarely maintained] this soup ends up slowly in the water supply.
Much of PEX does not meet the 150 psi rating at certain temperatures, therefore I propose a pressure reducer valve set around 50 psi, then the ex. tank which will handle spikes if properly sized, then a relief valve set at perhaps 125 psi to protect the ex. tank from higher pressures and to save your pex and to alert you of the failure of the expansion tank by its dripping. Then the T&P becomes the last line of defense.
If code offers no law for pressure protection, and PEX has ratings below 150 psi, the code has failed us.