I have vented to my attic and had no issues. It was in the winter, though, so too humid wasn't a concern. Unless you live in a high humidity area, mold shouldn't be an issue. At worst, you might have a bit of insulation that has to go, but not likely. Most homes' bathroom fans vent into the attic. Actually, that can be your only problem, because when I did it, my wife said she could smell it her bathroom, so the air found its way over there even though my attic has very good ventilation to the outside.