PEX is not truly flexible in the same sense that rubber EPDM heater hose is. If there is no movement involved, and there are no tight bends, PEX will
work well. You will have to terminate it using PEX fittings, either with a crimper or with the expensive and less reliable sharkbite-type fittings. Once installed and secured in place, PEX will last for the life of the
hull. For hydronic use be sure it has some sort of oxygen barrier, either vinyl or
aluminum, or you'll have problems with boiler
corrosion.
I have installed a lot of PEX and the only failures I have seen are a result of fasteners penetrating the pipe. It is soft and more prone to such damage than most other materials.