I use flashlights as bike lights all the time--it helps me feed my flashaholism by giving me another reason to "need" another flashlight! I have strong preferences related to tint, beam shape, CRI, price, and efficiency; and, find that I can scratch my itch better with the selection of flashlights than with bike lights. For single-cell 18650 and 21700 handlebar lights, the cheapest mounts on 'zon work just fine; for a helmet light, an over-priced mount from Exposure has worked for all of my 1" light bodies for years.
As with any powerful tool, don't use it poorly and ruin it for the rest of us by strobing or blinding other trail and road users!