Ive never ever had a problem jacking up a vehicle from the diff, theres nothing wrong with it. Some vehicles dont have a jacking point, so your only choice is to use the diff.
Sills are where you put the stands, because thats what they were made for, when you see a car being put on a 2 poster hoist, unless its a 4wd/ute and some vans, the 4 pads usually go under the sills.
I think aslong as you have safely rested the car on the stands, whether it be 2 or 4, and always do the shake test, i mean id rather see my car fall with me shaking it then fall with me under it. Ive never ever seen stands failim not saying they dont though.... and ive been using them regularly for a while. I guess it also depends on quality, its no use in being a cheap skate and getting some el cheapo deal, always go for what tradies use.
And i figure if you are too scared to get under a car, then just dont you start getting paranoid at the slightest creak or groan, i used to be terrified, thinking how the hell is that little puny jack going to hold up 2 tonnes of car.... but i got over it, you jsut have to place trust in the equipment you are using, and if not, just get someone else to work on your car.
but thats just my 2cents