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.
That overstates their unreliability. Such dynamos used to be universal before modern rechargeable batteries and worked well enough, I don't recall ever having to replace or maintain one. For dirt roads where mud gets everywhere, non-mechanical options are of course better, but simple dynamos perfectly are fine on regular city streets, country roads and dry paths. They're especially good for kids who can't be bothered to keep batteries charged.
I don't remember a period of my life where bottle dynamos have not been unreliable when it rains, and it rains a lot here (or it used to, anyway).
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.
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.