The only problem I can think of is batteries. An old rangefinder probably runs forever off of a button battery, whereas digital cameras suck up a lof of power. But I'm sure you already know to bring extra batteries. And maybe an extra memory card.
I carried my Nikon 5400 digital camera in my handlebar bag last fall during a 500+ mile ride on a gravel trail. It never rained, but the camera was not bothered by the bumps and dust.