As Dr. Jones has said, the Glock is a very reliable firearm. It has enough internal and external safeties that the gun will not fire unless the trigger is pulled or the gun has serious mechanical problems. Of those two choice the first is by far the most common reason for an ND with a glock.
Now the problems with glocks and safety are these:
(1) The gun won't fire unless the trigger is pulled. The external safety is on the trigger. Therefore the external safety does little to prevent the trigger from being pulled since most situations where the trigger is pulled inappropriately would also disengage the safety. This is why glocks require holsters which cover the trigger.
(2) Glocks have a simple manual of arms. Glock owners are helpful and often seek to promote Glocks to new shooters for just this reason. This makes them appealing to new shooters. However, Glocks require proper gun handling skills and firearms awareness which most new shooters lack. Glocks takedown process requires the trigger to be pulled which is potentially even worse for individuals with poor firearms discipline.
In short glocks are good guns, they are safe and reliable if handled and carried properly. They are also highly intolerant of mistakes and poor firearms discipline if handled or carried improperly. That is the big complaint. Yes your first and most important safety is between your ears, but redundancy is a good thing should that safety fail.
