Is there a situation a punch would be more effective than a palm strike?

Punches are safest for your hands when you aim for the soft spots of the opponents body. A good rule is conventional boxing punches to the body, elbow, hammerfist, or palm strikes to the head. A well aimed punch to the nose or jaw is okay, but avoid the skull.

When I was a green belt I was told by a senior black belt that a full fisted punch was a warning shot. Shokens (one knockle punches) harikans (tiger paw) finger strikes and palm strikes are for when you really want to hurt someone.

As I've gained rank and learned more and more I now echo his words as I now teach my own mid level students!

One suggestion for you is to work on all of your strikes. Learn power, accuracy and technique. You never know when a certain strike may be best. So always be prepared.

"I can't punch very hard, but I can do a palm strike with reasonable force"

lol, it's the same body mechanics

Punch breaks your finger without gloves, and palm strikes breaks that hard bone at the end of your palm.