This question is valid for both mechanical and hydraulic brake systems.
- The human hand has a threshold value for what a comfortable level of squeezing force is, which will decrease with rider fatigue. (See Chris' excellent comment below.)
- The brake system is not externally powered.
- The usual laws of physics are being obeyed.
I am not understanding how some brakes can be unequivocally better than others.
Fundamentally, brakes can be modeled as a lever. A brake setup with large pad clearance is either going to be weak or have large lever throw. There's no free lunch; one always needs to balance power, lever force, lever freestroke, and pad clearance.
For example, the Shimano Saint and Zee downhill-oriented hydraulic brakes use a high leverage ratio in the levers to create more pad clamping force. The tradeoff for this is their long freestroke, i.e. how far one must pull the lever from its resting position until the brake engages. Conversely, most SRAM hydraulic brakes use a "normal" freestroke and a generous pad clearance, resulting in their characteristically mild power.
Of course, there are many design factors involved such as dual-pivot rim brakes, Shimano's "Servo-Wave" and SRAM's "Swinglink" variable-leverage designs, pivot bearing quality, two- versus four-piston hydraulic calipers, or even plain old manufacturing quality. These help overall, but since high-end brakes all incorporate these features, I don't see how any specific brand's implementation could clearly outshine the others'.
I suppose the inspiration for this was me wondering "How can $900 Trickstuff (or other premium manufacturer's) brakes be worth that price when they have to balance the exact same physical factors as $200 brakes from Shimano et al?" The high-end brakes are reportedly better than cheaper ones in every way, but I am not seeing how that is possible.