In areas that the temperature changes throughout the seasons drastically, flashing cement should not be considered a permanent flashing detail.
For instance, in NJ the winter/summer temp variation provides significant expansion and contraction deterioration. In these areas, whenever possible a metal (aluminum, copper) flashing is much more effective for a permanent flashing detail.
If you are insistent on using roofing cement my recommendation would be to utilize a fabrich (mesh) inlay.
Hope this helps.
By the way, as cement goes, Karnak is a reputable product.