My understanding is that modern quality lights have a voltage limiter, so that they don't get destroyed from too much current. There's a certain speed at which a particular light attains its full brightness: above that speed, there's no change, below it, the light's dimmer. LED headlights seem to reach full brightness at lower speeds, so if you're worried about being able to see going uphill, you might consider an LED. I've never used a dynohub, but I've had bottle generators, which work similarly, and while the light is somewhat dimmer if you go very slow, I don' think it's ever been a problem with my LED fly IQ. I don't think it was much of a problem with the Lumotech either, but it's been about a year since that bike was stolen.

For one thing, since you're going slowly, you don't need as much light. Also, I get the impression that the hub generators are more efficient, which would seem to make this less of a problem. Finally, some lights (including the fly) let you adjust them as you ride (provided you mount them in easy reach). This is quite handy.

Regards,
Mike