Well, two problems.
1. Your Six Shooter is turned into a Five Shooter and your auto goes from MagCapacity+1 to MagCapacity-1.
Also, notice they do not demonstrate how to make a revolver "ready". It appears it would take working the hammer to cycle the Safety Round to the firing position, then pulling the trigger to rotate a loaded round under the hammer. I guess it would get a little trickier with a shrouded hammer.
2. The procedure to make the gun ready, for both revolver and auto, required what amounts to cycling the action twice....under pressure. Fumble it, or forget to do it at all in the heat of the moment and you are left with a disabled SD weapon. You can still throw it, I guess.
Giving up one round of capacity in a revolver and two in an auto is a big price to pay. The risk of inadvertently rendering your SD weapon inert is, to me, a major concern.
I cannot see using these in my "ready guns", but then I don't have kids in the house and ready guns are either being carried or very close.
I can see these would be valuable for stored guns, particularly if there are kids or other visitors in your home who should not have access to a loaded weapon. Leaving the weapon unloaded does part of the job, but having the gun disable itself with an "unauthorized" pull of the trigger could be a plus.
For me, my ready guns will remain fully loaded and immediately function-able without the risk of becoming disabled....and I will continue to carry them and/or have them within reach.