In my case it was the connecting tabs for various parts (though the arm-connection tab for at least some releases, like the one I had, were dicast, so that one was fine), plus Long Haul's head breaking off at the joint, and other 'thin' areas cracking over time. This happens in the KO's too, but a lot faster. Keep in mind, though, my expriance was with stuff that was DECADES old, so the modern one may give a lot better experiance, I really don't know.
Edit: I should also note re: yours above, which was also from the 80's, that mine were pretty heavily used in kids-play, I don't know what sort of abuse yours took, so my own experiance may not quite line up. If yours WERE heavily played with and still held up fine, I may simply have been unlucky. Toys DO break, sometimes in ways that don't quite make sense.