Messenger bags would not fair as well in hot weather due to the amount of of the bag is against you back becuase of the way it wraps around you, so back sweat would be a problem. Otherwise they seem to fair better then a backpack, seem more comfortable on a bike, stay put if you have it on correctly, and the reflective staps is a plus. Also, most zippers and the zipper tracks usally arent waterproof, and even those that are, could get screwed up when the teeth fail to match up correcty causing gaps which leads to a leak, which could pose as a problem, if you would rather the water stay outta of the bag. Messenger bags dont have zippers to worry about.