I'd say they're as safe as any other pistol. I consider myself very open minded, but I wound up not getting one, instead, I have a CZ P-01 that I'm pretty happy with.
After reading into Glocks when thinking about getting a G19, I read a lot about "Glock leg". I found that a prime cause of Glock NDs resulting in injury is reholstering with something not your finger (shirttail usually) stuck in front of the trigger. On a DA/SA gun with a hammer, you can holster with your thumb holding the hammer forward - if it moves when reholstering, you know you should stop. Also the DA pull is pretty heavy. With a gun with a safety, that's also mitigated as the gun physically can't fire while reholstering.
Some other reasons I didn't get a Glock were that you can't do dryfire practice with it (recocking every trigger pull gets old), and I carry in a Smartcarry so the gun is pointing very close to Mr. Happy. I've found that under stress shooting, I never ever have noticed the transition from DA to SA trigger pull weight. Also, the worse thing than having a gun go bang when it wasn't supposed to is having a gun not go bang when it's supposed to. In this video, you can see that Glocks limpwrist badly and repeatably, like if you were shooting wronghanded or injured.