Re mildew, I haven't had any issues but figure common sense comes into play. I haven't spent months and months on trips, but I do recall leaving the bag sort of open the few times for some reason I had damp stuff. In general Ive always left damp stuff on top of panniers, but the times I had damp things and it was raining, I put the damp stuff in a plastic bag until it stopped raining and then put them on the bike somewhere.
Traveling in a rainy season in a hot country would be a drag, but then your stuff is pretty much going to be kept dry with any dry bag style pannier, and keeping wet stuff out of a pannier is a pretty basic solution.