So, they put a supercapacitor between the rectifier and the output to stabilize the voltage. This is old technology. The reason why so few cyclists use a dynamo nowadays is not because of the voltage instability, it's because a dynamo slows you down, and because good, long-lasting and rechargeable batteries are ubiquitous.
If you're out for a couple of hours, batteries are grand.
I looked at the options and bought a $50 20000 mAh powerbank instead. Enough to charge the smartphone 4 times and the lights basically forever.
Dynamo is much more likely to break than the batteries, less convenient, and more expansive. It's obsolete technology IMHO.
I expect I'll need to service it in another few years.
In the meantime, I've gone through at least three reasonably priced battery packs; two because they crapped out, and the last because i wanted USB-C.
I'll take the dynamo hub any day for my bike lighting and power needs.
oh, and as an added benefit it's way less of a hit on the environment!