With us, an uninsulated attic, black tiles can reach 60°C in summer and -10°C in winter. Wine, valuable pictures, etc. should therefore not be stored there. Furniture, toys, clothing... are not bothered by it.
Exactly. Plus 60 and possibly minus 20 if it is really freezing cold outside with -30, but no more than that. I think a bachelor thesis, the child's balance bike, and plant pots won't be affected by that.
Temperature is one thing, humidity another. I wouldn't put furniture or textiles up there. Other things, which could similarly go in a garage, however, yes. I, as a flat roof owner, believe that for the additional costs incurred, a not insignificant, qualitatively improved usable space is created. Others dig expensively into the ground for that.