I had a ND today

[SideShow Bob]

Is it less hard on the gun to do the hammer follow test with snap-caps?

With snap caps you don't get the full force slam / jar to cause the slip on the full cock notch.

[Irving]

With snap caps you don't get the full force slam / jar to cause the slip on the full cock notch.

Then how did it happen with live ammo in the chamber? Are you saying it can still happen with ammo (obviously) but it's more likely to happen (and then be discovered) when empty?

[SideShow Bob]

I've had a stuck firing pin go full auto (luckily with only three rounds left in the magazine) due to a filthy pistol. And a slam fire from a protruding primer.
A hammer follow would be a mechanical problem that would have repeated itself while continuing shooting.
The empty chamber test is what should be done sparingly, usually after trigger work is done or for a suspected problem.

And yes, it is more likely to be found when the slide is dropped on an empty chamber.