Fog is an issue in January at least. On our boarding day, the ship was delayed by fog and people were not able to get off until afternoon. I think we arrived around 2:30 (can't completely remember) and people were still debarking. Our debarkation day, we were delayed a few hours. The drive from the Galveston port to IAH is easily an hour and a half without too much traffic. Could be longer with traffic, which is something we definitely have in Houston!
On the other hand, we did not have any problems finding our luggage nor did we have too long a wait to get through customs. We did wait until they called our luggage tag group--did not try to get off early as some people did. At Galveston, they bring the luggage out in waves, so if you try to leave before they call your group, your luggage won't be out yet.