A camera guy I used to work with had his Rock n Roller for over 5 years (but never got the shelf), he loaded a pelican rifle case at the bottom with at least three C stands and a baby c-stand, on top of that four 1650 pelican cases loaded with an Arri light kit, KinoFlo Divas Kit, Sony Video monitor, dimmers & stingers, v-mount batteries & chargers, arri & avenger light stands, schatler tripod, duvatine & gels, sand bags,etc... It never let us down. Thats why I decided to get one for myself, thinking that it could make a nice follow cart in the future when I can afford a Chinhda cart...
The shelf is made of plywood with steel beams bolted underneath. so I'd say it can hold well over 100 pounds, the problem is that the mounting holes on the handle bar might be too high for a twelve unit rack so you'd probably need to drill new holes. I use a six unit SKB rack, or just put my Sound bag on top depending on the gig...