I have seen it leak when it is applied to an area where the leak itself was still on going. That makes it impossible to ever get the area clean enough to get a good bond. I've also seen it crack when used in an area where flex of the repaired part is possible. I've used it to hold small studs in cross threaded or damaged thread areas and it's not very good in that application, depending on how tight the nut on the stud needs to be, it can pull the stud out. It is said you can 'drill and tap' a new hole using it, but thats never worked for me. I recently went with a heli coil over JB on a small engine case I was working on,,, but JB was tempting, again.
Crack in a trans case or an out board motor 'hole in one' is an excellent application for a non stressed area. Gas tank crack on my Jeep, it's been four or five years and holding well, of course I don't RACE the Jeep.
If it was Friday and were racing on Sunday with no chance for the part, I'd run it for the weekend at least, depending on the application. If I owned a boat, I'd never leave the dock without some on board!