Palm hand strike is shorter and the impact is on a larger area than the two knuckles, in addition the need to articulate the contact with the palm and avoid the impact with the fingers slows down the strike and therefore the momentum(mass times acceleration) is lesser.
With proper training you learn to control your punch and if you for some odd chance cannot hit the chin with the fist, you can limit the extension of the hand.
If you have to use it, it will give you distraction but you need to follow up with more devastating attack if you did not get stabbed with a knife.
Certain people that use their hands for delicate work are concerned about striking with their hands. There is an argument that palm is better for women.
Well, considering that it has less effect than a fist, but might be easier to execute for untrained or unconditioned person, it is sure better than nothing.
But to claim that a palm strike is better than a fist strike is just ridiculous.