My CamelBack really annoys the hell out of me on my road and gravel bikes, but I hardly notice when on my mt. bike. Likely as I'm leaning over on the road/gravel so notice it more. Plus and as other have stated, and for where I ride (wooded eastern seaboard) I'm in the sun more on the road, thus a backpack is hotter, not an issue as much in the shaded woods.

When I rode the Bike Tour of Colorado, I wore a small CamelBack that only carried energy bars and rain gear. I had stuffed my extra water bottle with required clothing, had one full water bottle (the rest stops were an hour apart). I didn't want to add a rack and trunk bag as it was heavy.