For light offroad use as described, they do great. Tires are the main factor. If you really are talking war era BMW they go well on gravel but my experience is limited to 2 rides. Very careful rides, fully restored bikes. Tires again would help enormously.
For more heavy use the big problems are ground clearance and weight. My budget plan for my 81 r100rt. Top brace, 1" over shocks, shallow pan and pickup froma /5, frame mounted bash plate, flip the exhaust, spark arrestor type mufflers, knobbily DOTS. Strip weight.