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The issue is that while compact fluorescents generate much less heat than incandescents, many of them are less tolerant of heat. On a hot day (40C), heating a fixture to 70C will only require a fifth as much power as heating it to 190C (374F). Although a well-designed compact fluorescent fixture shouldn't dissipate that much heat, some inferior units might.

So is the issue damage to to the light or to the fixture and surrounds? I saw this warning on a package of CF bulbs that I wanted to use in my bathroom IC rated light-vent fans but it's enclosed and according to the package that's a no-no. Yet I use an outdoor CF type which is the CF bulb in a weatherproof plastic housing in my outside coach lamp with no problems. I said screw it and bought a 60 watt incandescent.

So is the issue damage to to the light or to the fixture and surrounds?

The issue is premature failure of the fluorescent units themselves. Incandescent bulbs don't particularly mind high temperatures, but fluorescent ones do. Generally enclosing fluorescents by themselves won't be a problem, but with some inefficient ones it might be. On the other hand, putting e.g. one incandescent and one compact fluorescent into a two-bulb enclosure would likely result in extremely premature failure of the fluorescent.