I haven't ordered mine yet, but some other Rambox items:
1) There are two different utility rails, depending on whether the factory tonneau cover is ordered. With the order, the rails are black and the top is a box section that is level with the top of the bed forming a surface for the tonneau cover the rest on. Virtually no other cover will work with the black rails although I'm not sure about the Mopar accessory hardcover for Rambox.
The other is a silver rail that has a grove along its top surface. Some aftermarket covers that work with the silver rail virtually take it over. My favorite cover, the Peregon, doesn't work with the Rambox. The Retrax retractable cover has a Rambox version, but not with their convenient XR rails. However, like all retractables, it takes up some bed space and can't be easily removed. By comparison, the factory cover just lifts off.
Overall, many/most tonneau covers don't fit the Rambox. There are covers, but fewer options.
2) There is a very limited selection of ladder/item racks available, and some (most/all?) require the silver rail. Normally there are low racks available that extend over the bed with attachments like typical roof rails. There are also fancy ones for conventional beds that extend to the height of the roof and sometimes over the roof. An Australian firm showed a great one a couple of years ago, but it seems to have disappeared.
3) Similarly, there are fewer floor covers available. I am looking for something like the WeatherTech, but which can just be laid on top of the spray-in liner, not glued to the floor.
4) There are no stake pockets. A bit surprising, since there appears to be more than enough real estate on the sides of the Rambox.
5) The kits to store long items (e.g. fishing poles) are available for the 6'4" Rambox on HD models, on the 5'7" on the 1500s.
6) The Amp BedStep2 ( aside deployable step available on the F-150) does fit the Rambox.
7) As you know, the divider is in BUG and the extender pins no longer are included. There are questions if FCA will eliminate their mounting points (or if it already has since no one has tried on a 2020 yet.) It isn't clear if the Multifunction Tailgate works with the bed extender pins. I haven't seen the combo in person, and certainly not with the bed extender since that is no longer an official feature.
8) On the positive side, the Rambox with MFT doesn't have the corner braces on the front of the bed. Those braces are causing non-Rambox MFT owners Rambox like problems in getting tonneau covers.