...I reckon saddlebags, while undeniably appealing to a certain aesthetic, are a terrible way to carry Stuff on a ....
I feel I should defend saddlebags
As a rack bag needs a carrier, it seems only fair to compare them to a saddlebag supported on the same carrier: in which case it only needs the saddle QR bit of the bagman 2 (and a (toe?)strap through anti-sway loop) although, I have only ever used, on an upright with rack, the heavier SQR bracket , but modified to be 'up and out' rather than the standard 'down and up and out' (why did Carradice do that?). And carriers are a useful place to mount proper (large lit optic) rear lamps (I prefer to just use mudguards for auxiliary backup rear lamps
)
A Super C saddle has side pockets* can hold extra stuff under flap. A pouch can be attached to the top D rings. Most rackboxs seem smaller than a Super C or Camper Longflap.
On an upright the rider shields the saddlebag from rain a bit.
I found the 4 velcro straps on a Caradura Brompton rack-box a pain: and replaced them with one to stop rearward sliding (on Brompton's sloped rack) and a Fastex belt through the side D rings and the rack.
Perhaps rack boxes are better for tidy people who bring tiny amounts of food/gels, few or compact extra clothes, , hardly any warm clothing etc. I tried one instead and quickly returned to a Saddlebag.
*I suppose most rack boxes have side pockets too
but the overall capacity seems small and hard to extend to me.