The answer is training....lots and lots of training. The gunpig is not exactly a human factors ergonomic masterpiece. The AC-130H had the FCO and Nav on the flight deck but isolated form the pilots by a blackout curtain, the sensor operators and EWO were down stairs in a booth. The AC-130U has the NAV, FCO, EWO and sensors operators all downstairs in the BMC...yet somehow both platforms trained their way to excellence. I am not bashing the Buff...just one data point early in the war, I have a second data point but no need to dogpile. These are great Americans and they have since professionalized their contribution.
In the end the thing that typically makes the U.S. military stand out from the rest is not the platform but the people and the training. Train like you fight and fight like you train...joint CAS can be a beautiful thing.