The Beretta 92 decocker works by more or less firing the gun with the firing pin block safety engaged. Never heard of one of those failing and there are untold zillions of those out there.
The CZ52 works that way, and there are others. That kind of decocking system can be prone to failure with wear. It's generally not a good idea to use the firing pin safety as anything other than a failsafe. Using it as the go-to method for preventing a gun from going 'bang' during a decock seems to be a non-optimal design.
The Beretta 92 design is actually much safer than that.
1. It rotates the rear of the firing pin about 90 degrees which makes it impossible for a strike to the rear portion of the firing pin to transfer to the forward portion of the firing pin and forward to the primer.
2. As the rotation takes place, a piece of steel rotates upwards to protect the rear of the firing pin from the hammer.
3. The decocker trips the sear, dropping the hammer.
4. The firing pin safety acts as a fail safe in the extremely unlikely event that the initial two functions fail. Although I can't see how they possibly could. Maybe if there was some kind of really messed up situation where the decocker tripped the sear before rotating the firing pin--but I think someone would almost have to intentionally modify the gun to work that way.
The Walther PPK decocker is an inherently weak design. Not the fault of S&W. The design has basically stayed unchanged since 1929. I had an Interarms iteration and always assisted the decocker by using off hand to lower hammer while employing safety decocker.
I tend to manually lower the hammer when using a decocker as well, but that said, I'm not sure how the PPK decocker is weak.
When the lever is activated, it brings a cam up to keep the hammer from contacting the firing pin and then also locks the firing pin in place using the decocking lever to hold the "bulb" at the rear of the firing pin so the firing pin can't move forward or backward at all.
Again, I can see the possibility of the decocking function dropping the hammer before it locks & blocks the firing pin, but I think that the gun would have to be modified for that to happen. The lever locks the firing pin very early in its travel and doesn't trip the lever until it is nearly all the way to the safe position.
It is true that the Interarms PPKs (and I assume the earlier models--I can't speak to the newer S&W models) do not have passive firing pin safeties. It would be wise to carry them safety on to insure they are drop safe.