Placing the transition from clear to the ND portion accurately in the frame. On a 4x5 it is relatively easy to do this precisely, although in low light it can be hard to decide exactly where the transition is, which subsequently affects metering.This is more relevant for the stronger filters.On a workshop last weekend, a 35mm user was having difficulty accurately placing the soft filters and wished that she had gone for the hard stuff!
I only have two soft ND grads, a 0.45 and a 0.9 and use these in conjuction with a set of Hard ones when the image dictates. I also use a 0.45 hard one rather more than the 0.6 because I like the sky to be about half stop brighter than a true balance in most pictures. I feel that this offers a more natural look.