I think one of the most telling components of this video comes toward the very end. Note the massive amount of air leakage that is present at the air handler itself. This is the rule, not the exception. It takes a very careful installing technician to adequately seal the air handler of leaks.
When prioritizing duct sealing opportunities, we typically start with leaks that are closest to the blower. We do this because the leaks in close proximity to the blower are under the greatest pressures. Since they are under the greatest pressures, they leak the most, and we want to seal up the areas that leak the most. Not rocket science here.
I should probably take a step back. The first areas that we would want to duct seal are return-side leaks that create negative pressure fields in CAZ zones which backdraft combustion appliances. Then the ones closest to the blower.
Watch the video, as there are some good pointers there....