Do the crystals or beads work both ways, like the Boveda, hydrate and dehydrate, or whatever the term is? My understanding is that you can't over-hydrate with Boveda. It's "set" at the humidity level on the package. They do eventually dry out, become stiff, and then you replace.
As far as mildew or mold goes, that's why I don't buy large quantities of cigars. (Plus my tastes change drastically over time. And I hardly smoke cigars at all in cold weather.) I have lost cigars to mold. Not with the Boveda, though. Not yet.