Even in the attic or walls, sure it's sealed up most of the time, but any time you have to go in there to work you stir it up, then you get glass shards in the air which get in your eyes and lungs, so you have to wear safety suits and respirators and so on just to do basic maintenace work.
If anything ever goes wrong with it, it's a nightmare to dispose of.
Worst of all is using it to wrap ducts. The problem is that all ducts will leak eventually. Maybe not right away, but in 10 years cracks will form. Then the air return ducts will start sucking in at the seams, and eventually that will be sucking glass fibers in, then blowing them out all over the house.
Just say no to toxic shit in your home.