It takes a lot of trial and error to terraform a slope that peeps will use. As far as I can tell, they won't use a slope if the angles are too sharp at either endpoint of the slope. Therefore, it'd be nice if the tool told us when an outlined slope would be useable or at least gave us the projected angles so we could make better guesses about what will work.
Don't get me wrong, usable slopes can be built, but what a pain to get there...