What shape of wire forming machine is the most difficult to operate

A compound bend is "a feature in a line form that has two adjacent bends or builds without a straight length tangent to the start and end points of these radii." In simpler terms, compound bend involves making two The bends are so close to each other that there is no perfectly straight wire between them.

This type of bending is difficult because the wire forming process is done incrementally, with the wire going through the machine continuously without stopping. Compound bending is doubly difficult near the start of a length of wire, as a portion of the wire is usually required to begin the forming process.

In most cases, the length of wire needed to start the bend should be 1 to 1.5 times the width of the forming wire. This means that a 0.2" thick wire needs 0.2-0.3" of space to start bending. The 1.5x rule isn't always true, but it's a good general rule of thumb for making line shapes.
It takes a skilled operator to know the capabilities of the machine being used to reliably create such difficult shapes.