Condensation issues on bathroom or kitchen exhaust would be all but unheard of in Newport Beach CA, but in colder climates it's common. Sealing the duct seams & joints with duct-mastic, then insulating them to R6 where the runs are outside the home's insulation boundary reduces the problem. (In Newport Beach just sealing them would be good enough.) It's the dew point of the exhaust stream relative to the duct temperature that's relevant, not the outdoor dew point, but at SoCal temps unless you're taking a 1 hour shower at 4AM on the coldest day of the decade you won't condense enough to cause damage, and even then it would be a pretty minimal thing. It's only chronic condensation that becomes a real issue.
In MN in January the attic & outdoor temps are ALWAYS below the dew point of the interior air, let alone the even moister vent exhaust, and drips & dribbles leaking out of uninsulated unsealed ducts can cause all sorts of problems with accumulating water/frost/ice in the insulation & attic, etc.
Backflow preventers are important to limit outdoor air infiltration from adding to the cooling & heating loads in any climate.