@mpowers I have heard this question often. Independent research on UV light effectiveness in home HVAC systems seems to be non-existent. More information is available regarding commercial settings. Here is what I know about UV sterilization - it is dependent upon the light directly shining on the source of mold, bacteria, etc, the power of the UV lamp, and the length of time in contact.
UV lights are thought to be more effective when aimed at condenser, coils and other places where moisture may collect and the light hits continuously. But nobody offers statistics on the measurable reduction in mold or bacteria on the air flowing through a UV lighted duct, probably because one would need a series of lights throughout the ducts for there to be sufficient contact to have an effect.
UV lights must be always on to be effective, and bulbs lose effectiveness over time, needing to be replaced at least every year. UV light can degrade some HVAC components in just a few years.
Finally, the effectiveness of a duct system may be affected if you open doors and windows, introducing more pollutants, and if you don't run your system fan full time.
Overall, with the advice of our HVAC vendor, we have opted for a more typical airfiltration system, augmented by room HEPA filtration systems in the occupied spaces, as well as diligent maintenance.
Sue