That will give you the average of a slope. But I don't know of many hills that are consistently at one angle. On a 10% slope you will find parts that are steeper and some that are less.

Many moons ago I climbed Ventoux. 13 miles at an average of 7.67%---EASY.

But that did not account for the Km at 12% that followed the 2 Km at 10%.- nor the final stretch at 10%. And to be honest- The downhill bit in the middle (Or so it felt) when it slackened off to 5%