Water often adds a degree of difficulty to making the oil or butter. Water and oil boil at different temperatures, so it is hard to be sure what temperature your oil actually is. Using unsalted vs salted butter has not effected it in my opinion, but i would strongly recommend only using a double broiler method, and keeping the oil or butter separate, and don't add water to it. There also a number of things you can do to increase potency, such as decarboxylating your weed before you put it in the oil, or using an emulsifier such as soy lecithin. Crock pots with temperature settings also work well when working with oil