I just looked up a Sylvania electronic ballast and they say the max case temp is 70C. I think that suggests normal attic temps should not be an issue.
The attic temps will be well below 70C allowing the ballast case temp to remain under 70C
that might be true for a ballast that can dissipate the heat it creates within its self, but heat is an additive and any heat produced by the ballast including the lamps will add to the ambient temperature to cause a much higher ballast temp, we see this with transformers, one of the problems I can see is in most fixtures the ballast is inside of a metal can (the fixture housing) this will add to the temperature rise of the ballast as it will not be able to dissipate as much heat as it would if it was out in the open, add this to the ambient of the attic and you can have temperatures exceeding the rating of the ballast.
Not all attics will be well ventalated, I have been in some with dark colored roofs that were over 140? F and some even higher, while we hope this is not always the case, it is out there.
In the can lights I mention above we found that putting the box's around the cans did not reduce the heat but the direct opposite happened, it cause containment of the heat and Even more from the lamps, when we took IR readings we found we were getting 60? to 75? C readings on the ballast cases, the original ballast were 40? C rated which we found 60? C rated one we replaced them with which improved the failure rate but didn't eliminate the problem, it wasn't until we found 90? C rated ballast from Universal that we stopped having problems.
Have not had a call back since.