From our operational experience mostly in a northern climate, about 50% of the time when attempting to replace trailer brake pads, the magnet falls away and separates from the assembly. This renders the brake assembly useless, and the magnets are nearly impossible to re-wire properly. The mechanic is then trying to replace pads onto a non-functioning electronic brake assembly. There are many times when the electrical continuity of the brake wiring will fail before the brake pads actually wear out.
A way to confirm this is to test the voltage on each wheel while the brakes are applied. The brake pads can look good and be adjusted properly but when the brakes are applied... just don't work. This is an indication of failed wiring and continuity. So while one wheel brake assembly conducts an applied 3.6 volts, then next wheel on the same side may only show a voltage through the brake of 1.6 volts. This brake set will never work even though the pads look good because the electrical continuity of the brake set is failing.
So we have changed our approach to switch out the entire assembly every time. We spend more on parts when purchasing the entire 12” by 2” brake assembly when compared to just buying brake pads, but save significantly on the labor. It’s faster to swap out the entire assembly than just changing brake pads, and we don’t have to worry about rolling with an undiagnosed defective electrical circuit.