I had a visibly fried GE 100w HPS mag-reg ballast that I decided to try out for the heck of it and see what it would do, and it made a loud arcing noise with the lights in the house flickering and after about two seconds of it I unplugged it. The lamp never l had in it never lit up. It also welded the cord I had used to the terminal block. I never did see it arc either. The ballast was fried due to a bad capacitor, which had oil all over it, thankfully non-PCB.

Had a couple of electronic ballasts that went EOL passively, I think I salvaged the wiring and tossed the rest. As for magnetic ballasts, I had a couple that felt a bit sketchy when I ran it but nothing that really failed in a violent way.

I also got a couple of fixtures with failed ballasts that were dead before I got them, those I just tossed the blown ballast and put in a new one.

And finally I have a couple of used HID ballasts where the dry capacitor is losing capacitance. The cores are still good so I'll probably replace the capacitors if I put the ballasts in a fixture.

I've personally witnessed a number of T12 and T8 ballast failures.  The T8s were always the electronic.  I've turned on fixtures and POOF- the fuse inside blew.  Heard it.  I've also seen a few that failed when I had them on.  Have had a few Advance T8s and early generation Sylvania T8 ballasts that would smoke and get very hot when they failed. 

As for T-12 Magnetic.. oh man do those things go!  fire hazard.  Those tar burners would get very hot and a few would flash arc when they failed, the others just got super hot and dripped tar out the ends and when I would go to change them, there was heat damage to the fixture from how how they got.  And they smelled HORRIBLE. 

 One of the local schools had a pretty major fire a few years ago which was traced to an old ballast in a fluorescent fixture .

I've never had a ballast blow but I've seen a couple do it . I have had a capacitor release the magic  smoke inside my Norelco 180w SOX streetlight . Also , back in my teen years , I wired a 50w HPS ballast improperly causing the lamp to grenade into schrapnel all over my bed room !!  I wasn't allowed to mess with lights in my room after that one !

Oops   

Never had a problem here, the good old magnetic lumps here were built to last, I've known them still work after 50 years half submerged in water in the base of a lamp post 

 I've witnessed a fair few fluorescent ballasts go up in a BIG cloud of smoke ! Many years ago a classrooom in school had one release a big cloud of yellow smoke !     We left and finished class out in the courtyard !

  This one was well done when I bought the fixture : https://www.lighting-gallery.net/gallery/displayimage.php?album=5203&pos=82&pid=163404

One of my friends works as a fire investigator, and one time he showed me this burnt up 2x4 troffer from a site. I took a look at it and it looked like it was from the 60s or 70s (had Y-slot lampholders) and it looked like it was retrofitted to T8 electronic from T12 RS. The electronic ballast ballast was spliced into the old ballast wiring which was stranded. The wiring was also a bit of a mess with multiple splices. Generally in a conversion I like to terminate the new ballast directly to the lampholders like in a factory fixture but most people don't due to time and money.

Anyway everything on it was burnt to a nice crisp and everything plastic was melted, one of the lampholders was also burnt away completely too. 

Not sure if the ballast was the cause but it didn't look too budged. Plus electronic ballasts generally don't fail in such a violent manner.

I like to fuse my 2xF40 RS ballasts individually at 1A. It shouldn't trip under normal use but if the ballast starts to act up, you have a better chance of it just blowing the fuse.

Wasn't it just a victim of the fire that started something else?

I mean it could be, but I mean if it was taken as a sample there was a good reason for it. And I trust his judgement more than a opinion online...

Zim building at Tel-Aviv, catch fire as a result of a 2xF40T12 fluorescent batten with a stuck starter that overheated the ballast, and the building wasn't built and wired according to the standards in Israel: https://he.wikipedia.org/wiki/%D7%91%D7%99%D7%AA_%D7%A6%D7%99%D7%9D_(%D7%AA%D7%9C_%D7%90%D7%91%D7%99%D7%91)

The ballast in my PG17 turret fixture just blew 5 minutes ago and emitted smoke that smelled like extremely burnt French fries. Ballast is swollen too. Suspecting capacitor failure. The light wasn't thermal cutting out. But before the complete failure it started buzzing louder and louder. I was in my garage when it happened.

A lot with fluorescent ballasts but only one with HID. I saw some guy repairing a highmast, and he must have wired it incorrectly as sparks flew out with smoke. Thankfully no one was hurt, he turned it on from the base not near the lantern.