Most of the time when the fiberglass was used, it was applied over the existing rusty metal. because the rusted metal wasn't removed, it kept rotting away and often the fiberglass would simple fall out when the metal it was attached to rusted away.

These days, bonding the new metal to the body shell is the fix. The problem is if the old rusty metal wasn't removed and the new metal was bonded to it, it will fail just like most of the old fiberglass repairs did. If the rusty metal was removed and the new metal was bonded to clean existing metal, the repair should be as good as a welded in panel. Gene