From what I observe, most bag designers can design the front soooo much but back support or back ventilation roughly same pattern/type one. The worst is just one flat piece. Can even claim will keep back cool but logic already tell how effective it really is.
I avoid those flat back type like a plague.
Common ones with added padding to create air channels is better than none but still meh....
The better ones personally for me is those suspension system with ventilated mesh types but those is hardly available for urban type bags. I use to own a black Tatonka 20 liters for nearly 10 years. Used it for work, travel,in concert (smuggle a DSL in the bottom hidden compartment) .... basically everywhere.
Edit
Due to Tatonka wear & tear, I used
Timbuk2 Especial Medio
5.11 IGNITOR
northface kaban
Black Ember Citadel
axiom 18.
Not sure if its nostalgia, but so far all those packs I used just cannot compete. Hence been always changing.