While geometry and design play a roll, Heat treat is the most significant source of blade failure. The tang should always be tempered to dead soft.
Some claim the tang should be able to withstand 50 foot pounds of torque, that is more than you can apply with just your hands.

It is too bad that knives do not have bullseyes like firearms do to reveal their strength and quality. The way it is now we have to rely on the maker to do the testing because we cant go shoot a group to test the knife. The best we can do is ask the maker about his guarantee or buy one and test it to destruction if you wish.