It's not a safety. It's essentially a striker indicator that can be added to a Glock. And, Glocks don't have grip safeties.
It doesn't have to be manipulated to fire the gun, it's completely passive. If you don't put your thumb on it, or look at it, you wouldn't know it's there. It doesn't affect the function of the gun in any way, as long as your thumb isn't clamped down on it. The advantages of a striker-fired pistol are not lost, while the holstering advantages of a hammer gun are gained.
How many of you guys who voted "Nay" have handled a Glock with one of these installed? I feel like you haven't, because most of the comments indicate some lack of understanding about what the device is for, what it does, and how it does it.
And,
@dmarbell , the SCD is about 0.00001% more likely to cause a malfunction than the stock backplate.
Here's a short video that does a better job of explaining the advantages than LTT's vid above, from a guy that bought his own: