That's a bit of an oversimplification.
There's information out there based on pressure testing from independent sources other than Glock. It's not an instant recipe for disaster--the issue is that it can be unpredictable and even minor changes can result in significantly different results. If one is not paying close attention to the right things, the results can be a problem. A lot of people do it and get away with it--so that's certainly possible. Maybe they get a bore that is a little slicker, use an alloy that is really hard or keep their velocities a bit lower, or do any one of a number of things that might keep them from running into trouble. The problem is that someone else may try to do all the same things and get something wrong, or maybe just end up with a gun that is a bit more prone to leading and blow a gun up as a result.
For example, testing performed by an independent forensic engineer (See
The Glock in Competition) found that two apparently identical Glocks shooting the same loading showed dramatically different rates leading--one was building up lead in the bore at a rate 8x faster than the other. That's a pretty significant difference. That would mean, for example, that one range session you could shoot 200 rounds through a Glock, then shoot only 100 of the same rounds through an apparently identical pistol and end up with 4x the amount of leading buildup. Or maybe shoot 400 of the same rounds through a different apparently identical pistol and end up with 4x less leading buildup.
That kind of variation means that it's hard to come up with a recipe that says: Do X, Y, and Z and you'll be safe. Someone might follow instructions perfectly and still end up with a busted gun. It also means that what one person does and gets away with--even for a long time--isn't a guarantee that if someone else does the same that they will also get away with it.
It's also worth noting that detecting leading can be tricky for some people which makes advice to check for leading of questionable value. I've run into people who swore that they had barrels where the rifling was completely shot out but it turned out that there was just really bad leading. The already smooth contours of the Glock barrels can make this even more of a problem.
So is it a really scary problem that frequently causes guns to explode? I don't think it is--a lot of people do it and get away with it. But it isn't just BS either and it isn't just about the typical manufacturer recommendation against reloads. It can and has verifiably caused some catastrophic incidents over the years. I just see it as something to be aware of--it's always a good idea to make informed decisions.
