Never used this on my own boats, but I have rigged a couple of club boats with it, and they had no problems.
(This is assuming it’s rigged so that the hook is the only thing touching the clew so it takes both leech and foot tension, and not with a separate clew tie-down line or strap through the cringle.)
I have a hard time imagining what would unhook it IF you have the vang rigged right. That is, it’s under tension even at its loosest. Tie a stopper knot (or a bowline handle) at the vang cleat so that the pin-to-pin distance between the mast tang and the vang key doesn’t exceed 63 cm. (I just noticed the other day that Roostersailing’s recommendation is 25 in (63.5 cm), but he measures to the point where the key touches the boom fitting.)
So just having the vang attached should mean that there is constant tension on the hook, which should keep it from disengaging...
In what kind of situation did your unhooking happen?
I don't remember exactly.... Probably operator error in setting the hook, as I usually have plenty of vang on. What happened after it unhooked is of course the boom dropped off the gooseneck and I had to one-hand grab the foot of the sail about halfway and "become the boom and outhaul" in order to sail back to shore. My arm got exceedingly tired. (It was too windy to effect the hook repair out on the water)