My "scary failure" with the Dunlops involved a fairly clean 1958 Les Paul Special. By this point, I'd used Dunlops for the previous 10+ years, and I KNEW that the pin was seated and checked it. It just slipped right out- I just happened to have a good grip on the neck.
Years before I tried some Schallers, the nut on the strap end came undone and I dropped a 65 Jazzmaster (no damage done) and swore off Schallers.
It wasn't until the Dunlop failed that I looked at the way they work and I think the Schaller is just a better unit. On the Dunlop unit it's either 2 or 4 spring loaded bearings that are bearing the weight of your instrument. There's too many things to go wrong with that system- if there's gunk in the trough in the female end of the straplock, if the shaft wears on the male end and it's cocked crooked- the bearings may not seat properly when weight is on the unit, a bearing could fall out...
On the Schaller system, the strap button sits in a cup, and is held there by a spring loaded plunger. Even if the spring loaded plunger fails, the button is still held in the cup by gravity. The two weak spots in the system are the nut that hold the strap end on- a dab of loc-tite or clear nail polish will keep the nut from coming off. The other thing that can happen is that the button will wear down so that it doesn't get caught in the cup. Hoever, that's noticable- and I've been using the Schallers on all my gear for the past 6 years or so, and have evidenced no noticeable wear on the buttons.