This is a real problem. I have seen even new wiring destroyed from the heat of a fixture. In older houses I regularily reject fixture installations where an old fixture is replaced. the problem usually exists where the new fixture is enclosed and mounts tight to the cieling. any fixture with a pendant or the older open bulb types with an open shade did not transfer much heat to the box wiring. Even compact flourescent fixtures have destroyed the boxes and wiring from too much heat.
the second problem arises where new fixtures are required to be bonded and the box does not contain a bonding conductor.
