Interesting post! Back in the day I used a lot of M bulbs as the high school newspaper photographer. I graduated from HS in 1961, so you can see this was a LONG time ago. Of the probably hundreds of little and big bulbs - M5, 25,etc even some AG-1 babies - I don't recall one bulb exploding, ever. The bulbs gave their all in silence, although there was an occasional crackling sound as the bulb cooled. While I never did tests, I seriously doubt that the envelope of the bulb got hot enough to start a fire. These days I smirk when films and TV shows present a Speed Graphic or such firing a bulb with a loud sound. Never happened to me! Now flash powder must have been a different story.....
While the brute power of a big 5 or 25 was welcome shooting football action at night, I certainly don't miss using GNs to preset exposure for a fairly narrow zone where the action MIGHT occur. I would have killed for an automatic electronic flash back then.... I counted a night football shoot a success if I got five or six good shots.
I definitely notice a loud pop sound! And having been in several rock bands and worked at several manufacturing plants over the years, my hearing is definitely pretty bad. But they're not that much louder than most electronic flashes. Certainly not on the level of what some old movies make them out to be.
The protective coatings on them are getting older, so they don't hold together as well. So the odds of one exploding and sending shards of glass out are higher now than they once were. Though to be honest, I haven't had one present a problem yet (but I don't use them often either). I've had a few cauliflower out to where you could see on the insides, but the coatings still held the glass shards together. I've heard stories of people getting glass in their eyes and face from those old bulbs though. But that's mostly second hand stuff. So while it's rare, it can happen. I think standing about 10 feet back should be safe enough.
And the heat isn't likely to start a fire either under normal use. But once again, it depends on how soon you pop the bulb out, and what you pop the bulb out into. They're filled with shredded metallic zirconium which is used in fireworks and as primers for cluster bombs and stuff. Think about how much light they give off, and realize they give off more heat than they do light. Like I said, my subjects can feel the heat off of one standing 20 feet back. They go from room temperature to crazy hot in less than a second, and then cool back down pretty quickly. So if you take a shot, let the bulb cool for a few seconds before popping it out, and throw it away in a metal trash bin full of wet food, you shouldn't have a problem. But if you immediately pop it out into a bin full of oily rags, you could have a problem. Two extreme examples, for sure. But my point isn't that it's likely to happen, just that it can. So it's worth keeping that stuff in mind, because those aren't usually issues one faces with electronic flashes.
Another thing to be weary of is the bulbs going off randomly due to static electricity. So they advice you not to keep bare, unused bulbs in your pockets. They active with just a few volts, so it doesn't take much for one to go off, and if it does while pressed against your skin, it can scar you.
So yeah, people have used them for decades, mostly without incident. They're not death traps or anything. But they do require a bit more care than your modern hotshoe electronic flash.