I don't know if it is a con per se but the trans operates at full line pressure all the time. Some extra wear and tear on the pump and drive plus it generates more heat so a good cooler would be advised.
Kevin
I've driven over 7,000 miles with a manual valvebody using a Hayden cooler in front of the radiator. Never any issues with pump wear or fluids. We put a lot of power (between 600 and 850 depending on spray level) and no problems. I keep a close eye on it and pull it apart every couple of years as preventative maintenance.