Thanks for your input so far.
I removed one of the "sheetmetal" screws during installation yesterday to locate the roof screw I was supposed to use in order to dial in the exact hood location I needed to match. The sheetmetal screw still had caulk in the threads so assuming all of them were caulked I'm not really concerned about them backing out. And, they have held the Maxxair hood on there for almost 3 years.
My bigger concern was the hood being connected to the square plastic flange of the fan instead of screwed into the wood (I assume) under the rubber roof liner where the fan itself is mounted.
The point about plastic becoming brittle after a few years is something to think about.
