Because of the hammer block, the S&W revolver with the hammer mounted firing pin (FP) is safe if dropped even if all charge holes are loaded.
In theory, the new frame mounted firing pin (FMFP) could be argued to be less safe from a drop on the muzzle than the hammer mounted firing pin for the same reason the inertia firing pin on a Pre-Series 80 1911 is less safe.
The FMFP is, in its simplest terms, an inertia firing pin held from the primer by only a spring. The cross pin and slot built into the FP allow enough forward travel so that the FP can strike the primer, so the slot and cross pin are not really holding the FP from striking the primer, in the sense of preventing a discharge. The hammer mounted FP, on the other hand, being attached to the hammer, is held from the primer by the hammer block. This makes the hammer mounted FP safer, in theory.
The FMFP is, however, so light in comparison with the comparatively huge FP on the 1911 that it would take a drop from a considerable height to cause an inertial discharge. This same method of using a very light firing pin spring (titanium) in combination with a strong firing pin spring is used to good effect to prevent inertia firing on Series 70 Colts made today and in the new E-Series S&W 1911s.
So, to answer your question and the comments of a couple of other posters in this thread, NOTHING stops the FMFP from striking the primer except the FP spring. In the FMFP design, the internal hammer block really does block ONLY the hammer, not the FMFP.
In my mind, this is not a big worry as Ruger, Charter and others have used this type of FP for decades without incident. Certainly in today's litigious society, if there were a problem, news would spread like fire.
