Opinions differ .. I have a collection of Audio cables, a pre amp , electronic tuner , Music books in binders , and can even stow a Folding Music stand In it.
It's been more useful as a shoulder bag off the Bike than On.

the Messenger bag I got in the 80s has a waist belt , I wrap it around the tubes of My Brompton bag Support frame , and have the frame to strap Boxes On

and wear the Bag, Or usually have the bag attached to the frame and take them off together .. not wearing It to ride