What is the flow range, or more specifically the pipe size? Typically PVC will be the cheapest option due to labor savings. And with current copper prices, probably on matrial too. But PVC material gets expensive on larger sizes.
You also have to consider the fire rating of the pipe as it passes through any fire separations - even if it's filled with water, the Code Officials choke on the flame and smoke spread rating of PVC, so if you are penetrating fire separations, you may have to go back to metal pipe for the penetration then back to plastic.
Another issue is the solvent welding of the pipe, an incident occurred on one job I know about where the fumes from the solvent were flammable and touched off an explosion inside the pipe when a local spark near the open end of the PVC pipe ignited the fumes that were trapped inside the pipe. This was in a length of about 100 meters of 300mm schedule 40 PVC pipe. So if you do use solvent welded joints, make sure the Trades ventilate the pipe during assembly.
It's still a fair cost saving vs steel pipe, even if the steel pipe is mechanically joined (grooved locking clamp joints).
fire walls, I guess it depends on the jurisdiction.
Sometimes for the 2 hour or less separations I have seen it insulated with foamglas then fire caulked, going through a 4 hour wall required the transition to black, among other things.
PVC seems to like expand more than black
Take the "V" out of HVAC and you are left with a HAC(k) job.
I have seen too many PVC chilled water pipes blow apart and flood the building. One of the buildings was repiped in copper after multiple failures. I would not reccomend it, even though it is the cheapest option.
PVC piping is illegal for compressed air and whether it can be used with pressurized water (as opposed to DWV) depends on local building code. In some places, it's illegal due to the rationale that someone driving a nail into a wall or sawing can more easily puncture (w/o noticing) a plastic pipe.
Also, PVC has a high glass transition temperature (~90 F), i.e., brittle, so for chilled water, PE tubing or PP piping would be a better plastic.
As to toxic smoke, burning PVC creates hydrochloric acid, chlorinated dioxins, chlorinated furans, phosgene, etc. Don't see cyanide mentioned, but the others are nasty enough.
RO water and copper are a disaster here. Some black iron, but by far the majority of the chilled water piping is PVC, no freezing problems here.
I live in a salt corrosion environment and PVC is the pipe of choice here for cold potable water, CPVC for hot. RO water will destroy copper hot water lines in short time.
A lot of steps are made to keep it inside concrete walls, or kept boxed inside of stacked washrooms in building cores to keep it out of plenum spaces.
Take the "V" out of HVAC and you are left with a HAC(k) job.
