"Well, for example: locations in the mortal world are not always fixed relative to one another. Locations in Fairyland mostly seem to be. So do you know how the stability of gate locations will be affected by the fact that this planet is constantly rotating and orbiting its star, which is moving relative to other stars, whose own planets also rotate and orbit them? If gate locations are straightforwardly stable relative to planets but not relative to smaller objects, how small can a planet get before it stops capturing gates? That sort of thing is best tested in some out-of-the-way system where if you lose hold of a gate it won't have sunk halfway through an inhabited planet before you can get back to Fairyland and close it from the other side."