The shelves were made to support even large monitors. You can see in our gallery at www.ninjastandingdesk.com various shots of the 23" monitor being supported without the shelves overly flexing. Each shelf will comfortably hold up to a 27" monitor or iMac.
Each shelf is fabric with hardboard inserts (what they make clipboards from) and T9 military grade aluminum pole supports underneath. We have tested it to hold over a hundred pounds, our 30lb recommendation is merely what the shelf supports with minimal flex.