I may be different but I do not find flat bars comfortable after 10-15 miles. Too few hand positions, even with bar ends. Thus I prefer the drop bars on my gravel bike, I can ride for hours with a variety of hand positions to vary up my upper body position. It's certainly possible to take a non-suspended mt. bike, add drop bars and it'll be a terrific bike for dirt and gravel roads and trails. Or buy a bike designed for the role if you don't have mt. bike without the unneeded front suspension fork.