Not having to charge anything or worry about batteries running out in the dark is a real benefit.
It's all on the very opposite end of the cost spectrum than the cheap bike light in this post though
They're just generic AC motors held against the rim, as simple and cheap as it gets. Nowadays I'd add a basic voltage regulation circuitry to get a stable 5V DC out of it.
You'll spend more time futzing with one of or dealing with weather-related failures that you would have saved more time charging a pile of batteries before every ride.
I have rigged all kinds of rubber contraptions to make them work better, I have made my own dynamos from old electric motors as a kid, and yet I have never in all those decades found a way to be sure they would work when I need them, never.
I recently bought a dynamo from what should be a good brand (dunno the name off the top of my head). Cost about 45 Euro's, not cheap. Just after a couple weeks, whenever I cycle through the rain my dynamo wheel would slip and I'd be fully without light. I thought it was not properly pressing the rubber of the tire. But I found that the wheel was just cheap plastic garbage, totally worn flat already. I guess capitalism flew by, enshittifying once more what was once good.
The old-school dynamo's were undestroyable iron casings, with an iron wheel with deep grooves. Now I am looking if I can find one of those again, to last me a lifetime.