First and foremost, the price is $199, which is about $50 more than the R&R R-10. Additionally, it has solid wheels, which are not forgiving when you are wheeling gear over a rough surface.
Next, it does not fold as flat as the R&R, therefore, taking up a bit more space in the van. The handles are round, which ins't conducive to attaching a top tier. The R&R handles are squared, which made adding a top tier very easy and inexpensive.
While aluminum would be lighter, it would not have the structural integrity of steel. Consequently, it would be about 50-percent bulkier.
There is a way to make one-half of the bottom platform of the R&R into a solid platform with the addition of simple aluminum sheeting pop-riveted in place. You could also use 1/4-inch thick Luan plywood in place of the aluminum sheeting, which would be quieter and easier to work with. The difference in weight would be negligible.
I've had 2 flat tires with the R&R during the past decade. I fixed the flats with Slime, the green stuff used for emergency tire repairs. In one instance a long thorn pierced the tire. In another case it was a roofing staple. While solid tires would prevent flats, the vibration on sidewalks and mcadam surfaces would be horrendous.
I don't see the above cart as a viable replacement for the Rock & Roller R-10, but that's only my point of view.
Gary