In the modern day "tech" harvesting scenario, I've always considered the grain cart operator job, to be the most demanding? 

On-the-go loading position/speed/cb communications with combine operator(s),

Catch the combine before its grain tank overflows or that machine stops,

Extend cart's unloading auger while approach truck, mindful of truck exhaust stacks,cab,

Unloading onto truck, be mindful of cart scale, don't overload truck axles, 

Fold cart's unloading auger into transport position before proceeding back to combines, 

Be mindful of uneven field topographies(swales,waterways,gullies,washouts,etc) to avoid/minimize damage resulting of combine unload auger hitting cart & visa-versa, 

Get out of the way at the turnrows/wide sweep turn away/then get back to combine, 

Keep track of total bushels loaded out of each field,

etc.....

Works well if combine(s) stay together within a conducive harvest zone where the combine's unloading auger(s) are always approachable for grain cart(s).

Every other component is waiting for & relying upon the grain cart's performance.