The boxlock is simpler, therefore easier to regulate than the sidelock with its 19 bits per side.

The Model 21 Winchester simplified the boxlock even further, doing away with the V springs, using coils instead. The V springs in the boxlock are A- hard to make and fit, and B- they drag on the top of their slot when firing and cocking.

However, to return to the original posting. There are simple sidelocks, like the Chech BRNO. A gunsmith friend has one that has been used hard. The greying edges and the dulled barrels give that gun a soft overall look that would qualify as a simple gun with plenty of character.