If done properly the pig dies nearly instantly.    Same as any other euthanasia method done properly.

The goal should not be hitting the head.    Ideally, you break the neck.

Every method of euthanasia has pros and cons.   Whether it be cost, safety, time, etc.     In a perfect world every sow would give birth to 10 or 11 even sized perfectly formed, healthy and energetic pigs.

We don't live or farm in a perfect world.