So if you hold your thumb on the striker indicator it will not fire even if the trigger is pulled?
The striker indicator has to move backwards for the gun to fire. If you have a good hold on it, it shouldn't be able to fire with your thumb over it. The only way it could is if you let the indicator press pretty far into your thumb. At the worst case, you would have a lot of warning of what was about to happen, and probably some pain in your thumb depending on the gun.
The XDm is a pretty sharp indicator, and the PPS pushes out pretty far too. The Ruger SR indicator is more rounded, and I don't recall exactly how far out it comes. If you're paying even the littlest bit of attention I don't think you could have a ND with these guns while holstering, if you put your thumb in the right spot.