Sorry, I'm no HVAC expert, but in the many flex shops I've worked with
I've run into a few of the issues you've been warned about.

Moisture (high humidity levels) can migrate upwind at a surprising rate.
Without a physical barrier it will reach into all parts of the exhaust
system.

Condensation can occur where you didn't plan for it to (usually at
joints in the ductwork), and after reacting with other chemicals in the
air flow, leak out over time.

We had one mixed exhaust system made of epoxy/glass that literally
disintigrated  (poof!) when persulfate/water condensed inside and etched
away the glass. The replacement was PVC.

In another case, flux fumes from a roll solder machine, combined with
moisture in a common exhaust system to make a mild acid solution that
condensed and dripped out one of the velocity probe ports in the duct.

One system, installed in a clean room, was not interlocked properly, and
when the HEPA blowers shut down for whatever reason it would draw dirty
air in from the HEPA returns. There was also a problem with the power
exhausts on the photo-imagers pushing too much hot air into the common
exhaust duct, causing the fan to trip off.

A little less directly related to your question, we had one exhaust on a
hot oil refold that seemed to be slowly losing draw (over a few years).
When we finally decided to check it out, we found that the fan blades
were literally corroded down to the hub.

Good Luck,

Andy Magee
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