Sloping ground testing is done on carefully calibrated concrete slopes (Pax River and Boscombe Down).
Have you ever seen a sloping piece of concrete anywhere else?
Limits take no account of slippery slopes, high vertical CG and so on. It's a place where every helicopter pilot become a test pilot for a short while.
I've heard of a helicopter that had a sloping ground limit of 3° - when I asked the company said it was done at the worst lateral CG with the maximum wind from the side... (I got them to change the test method).
So, it's another case where we measure with a micrometer, mark with a chalk line and cut with an axe.