Good question. Strength of the lock, or strength of the action itself? I like the looks of an under lever on a classically styled gun but, I think the fewer holes along the action bar, the stronger the action. Among all of the various opening devices, the side lever reigns supreme in the looks department, for me. The question of strength would still apply. It's all theoretical as I'm sure no action has really failed due "only" to the opening/locking mechanism with proper loads.