When you buy an across-the-shoulder/messenger bag/man bag, the capacity of the bag suggests that the bag can be filled with quite a lot of gear. Indeed, a messenger bag can be filled with a comparative amount of gear to a backpack.
A backpack is essentially a mini-rucksack and as we all know, it is not all that difficult to carry quite a heavy rucksack with ease. The key lies with design. The functional, ergonomic design of a rucksack enables the human body to lug a heavy load with no adverse side-effects.
What about messenger bags?
The design suggests that it can do the same job as a rucksack/backpack. After all, a messenger bag has an equivalent capacity... Yet, the design is flawed. If you carry anything more than a threadbare amount of gear, it'll harm your body. The bag is sold on the basis of convenient access. You don't have to remove the bag to get inside.
I have clicky shoulders from years of taekwondo and Sifu is always encouraging me to be careful with how I 'use' my body. I'm very cautious when using an across-the-body bag. If I'm shopping; supermarket or town, or walking anywhere, I use a backpack.