I keep most of mine up in the attic and have done so for years and years. I have an attic fan. I haven’t had any problems.
I consider the hot car a bigger problem, if they’re getting direct sunlight, they can warp, I’ve seen it happen to other people’s stuff. In the 80’s, I remember cassette tape cases permanently warping in my dad’s car (although they looked pretty neat, it was like some kind of modern sculpture).
My stuff’s in the attic cause the room I want to use for storage in the basement is not currently climate-controlled. And unlike the attic, it is not dry, so mold will grow on stuff. I’ve read that the plastic storage containers we use are not air-tight, even the ones with locking handles, unless they’re labeled as such. And as I recall, air-tight storage containers are prohibitively expensive.