Are these products actually effective though? I mean the duration and intensity required to kill viruses in moving air must be quite high? Or is it just snake oil, like how a lot of hand dryers have bluish/purple LEDs (which emit no UV) that shine on your hands?

Yes, they're effective. But near-UVC is dangerous to your skin and eyes, so it absolutely must be shielded. Far-UVC is quite safe to humans, and just as effective against coronaviruses, but it's rather expensive at the moment.

https://www.researchsquare.com/article/rs-25728/v1

I can’t speak to their effectiveness in air, but they work very well for e.g. bacteria in rapidly flowing water. Aquariums have been using them for as long as I can remember.