Generally, when a ballnut is recommended, it's not because of the thinness of the crack, but the shallow depth - something too shallow for a cam.
Perhaps as likely is that the recommender happened to place a ballnut, and other options would also work?
I think Marc is right about shallow
vertical cracks being more receptive to Ball Nuts, because the Ball Nut width is narrower than the head width of most cams, which would have to be placed in the bad perpendicular-to-the-rock orientation. But for horizontal cracks, I think one might, in general get a cam in a shallower placement than a Ball Nut.
Ball Nuts certainly won't go into knifeblade cracks, but the large and medium sizes compete with the smallest cams and are potentially a lot stronger, and the smallest size is down there with little brassies. It is true that Ball Nuts are very finicky to place, but the fact that a very small cam goes in easier and seems to stick when jerked on does not mean that it is any good for a fall either, and small brass placements, especially with the newer, (and I think over-hyped) offset shapes, are every bit as finicky.
When you start using any type of small gear for pro, you typically try for multiple placements and resign yourself to climbing in a zone that does not include comfort in its designation.