With Flex being so inexpensive is it really worth Aeroseal the old ductwork? If you already have a well designed metal ductwork then sealing the existing may be worth it. Odds are the ductwork wasn't installed very well to begin with and doing the corrections manually is the way to go. Poor distribution/insufficient airflow is a bigger issue than minor leakage IMHO.
Duct leaks are never "minor". I see no reason whey they are not essentially air tight. Plumbing is watertight, why can't ducts be similarly tight? Most existing codes require ductwork to leak no more than about 100 CFM which represents about a quarter ton of cooling. If the energy bill is $2400 per year that totals $6,000 over a 40 year life of the system. For a typically bad duct system that leaks 400 CFM that is $24,000 over its life. How can this be ignored? I talk to installers who get systems down to 5 CFM leakage regularly and they say it doesn't take much time once you get used to it.
It seems as if Aeroseal has an important application where ducts are built into the structure and major renos are required to get access. Even when using Aeroseal, it stll make sense to seal the registers because flow levels directed at those registers needs to be above a certain velocity to move the rubber material to the leak otherwise it will just settle on the duct when the velocities get too low. Great for areas where velocities due to leaks are greater such as building cavity returns.
Aeroseal does make sense for difficult to access ductwork. For easy to access ductwork it makes more sense to repair it mechanically, while at the same time fixing those rooms that never seem to be cool/warm enough. If it were me building the house I'd have the equipment and ductwork all in conditioned space to begin with. That alone will add 20% to the systems capacity.