I live in San Antonio TX area and it gets really hot and sometimes really cold here. I recently bought an older house (1980's), which has composite shingles and R-19 fiberglass batt insulation in ceilings.
I have had two foam installers look at the house and both have recommended a "sealed attic" approach with all vents (soffit and gable) being sealed up. They talk about the attic becoming a conditioned environment which would also help avoid heat/cold transfer issues for the duct work and air handler which are located in the attic.
I am concerned sealed attic concept may cause moisture issues and is contrary to past wisdom of a well-ventilated attic.