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Used mine a bunch off road in the dark and it was fine. Did plenty of overnight rides, winter evening laps at the local trails, couple of Bearbones events etc. Good or bad conditions, never found myself wanting more light.
As with battery lights, pair dyno light with a helmet mounted light for anything fast/technical/lots of low branches (so you can point light where you want to go) or you're trying to open farm gates/negotiate styles etc.
800 Lumen at 17mph.
Not sure how useful that is for riding off road in the dark.