I would have figured the exhaust would be hot enough to melt the plastic. My understanding was that you have to have metal for the first X feet because that’s what would be where it’s hottest.
But what do I know? If they say you can use plastic from the beginning then have at it. It’s cheaper and easier.
High efficiency systems run a lot different. The exhaust doesn’t get even remotely hot enough to melt them. Similar, there are gas fired water heaters that exhaust through PVC as well. Newer houses don’t even have a metal B-vents/chimneys anymore since everything just vents out the side of the house on their own.
I have a tankless which is supposed to be super efficient but I don’t remember what it’s exhaust piping is. I know the stuff coming out of the house is plastic as well, but I don’t recall what the piping coming out of the unit is.