Thousands of competition shooters reload for the Glock including many of the top sponsored shooters in the world, all without incident for many years. Reloading issues for Glock are no more frequent than for any other brand, care must be taken regardless of brand or caliber. As stated earlier all brands warn not to use reloads, for 2 reasons, 1: to cover their ass and 2: to sell factory ammo.
FYI Read this link and many other references on the web where it is repeatedly stated that Glock intentionally has designed the gun with a loose chamber "to enhance reliability" - but the implication is that "you should not reload for these guns"
"It is often asserted that Glocks suffer catastrophic explosive failures (kbs, or kabooms) more often than do other guns. You can’t search long on the Internet before finding a picture or video of an exploded Glock. The question we must ask is what really failed? In nearly all cases of kabooms, the use of poorly reloaded and/or high-pressure ammunition is to blame. Glocks have a loose chamber that promotes feed reliability. As a result, they should not be used with reloads and definitely not with lead ammunition because lead ammo fouls the polygonal barrel and results in higher pressures."
As for your thousands of shooters, etc. if they are using a stock Glock barrel, I guess they just haven't won the "Glock Kaboom lottery" yet. If they have upgraded to a Lone Wolf barrel (as many do) They probably don't face the risk.