If the jackstands are good quality, properly pinned, rated for more than the weight they’re holding, and on a level and solid ground (like a level secion of paved driveway) than they’re perfectly safe. The biggest danger with stands is when improperly used, like cheap stamped metal stands on grass.
Personally, I like to back them up with the jacks, but in all honesty I don’t think it’s necessary if they meet the aforementioned conditions.
When working under only one end of a vehicle I prefer ramps, again properly used on a hard level surface.