Like tsl said they tend to flicker at slow speeds. How slow depends on the dynohub and wheelsize. I have a Shimano dynohub on a 20" wheel, with a Lumotec CYO N Plus headlight. It flickers below about 3 mph but the dynohub is designed for a 700c wheel. The CYO has a stand-light, but it's a lower intensity than when powered. I think that's common for front standlights.

I love the setup, only wish it were on my commuter now with its 27" wheels instead of my rando bike (recumbent).