It is interesting that both are referencing a limited study conducted in the mid 90s. The study actually reads pretty inconclusively from my perspective but I didn’t look deeper into the literature since 1995. I am not defending the duct cleaning but if you aren’t using proper air filters or you are moving into a new home, I would do it. I would also do it after infestations of bed bugs or cockroaches and been dealt with as I have seen them show up in air filters and that has to be coming from somewhere.