For what its worth, pex is not uv stable. installation allows for 60 days exposure to uv. The manufacturers intend for the product to forever live in the dark, as in enclosed within wall cavities. Exposed runs along even basement ceilings, where there is very little sunlight, are required to be covered in pipe insulation to maintain warranty. If you're concerned about crimp rings rusting, do away with them entirely and use uponor/wirsbo Pex A. Uponor Pex A has memory and is an expansion type connection as opposed to a crimp type. the pex tube is cut, a collar is slid onto the tube, the expander tool slides into the tube and expands the tube and then the tube is slipped over the fitting returning to its natural size and forming its own compression. Uponor has all manner of low lead brass fittings. Only uponor has Pex A, that is pex tubing with expansion memory. All other brands require crimp rings
As for glass filled plastic fittings, they are pretty tough little items. I would still use brass onboard, because its a hatteras and we do things belt and suspenders style, but to eschew glass filled plastic in a home is just to ask for higher costs with virtually no return. as for freeze up protection, pex will expand to accomodate a bit of freeze and cast copper, tube copper or cast brass fittings will break from freeze before the glass filled plastic will due to the overall design of the system.