Silver fluoride is a powerful oxidizing agent which reacts violently with water releasing ozone and fluorine and forming hydrogen peroxide. Even though it is light sensitive I think this explains why it isn't used. Even though fluorine is in the same family as chlorine, bromine, and iodine its chemistry is very different. This is true for the rest of the elements in the first row of the periodic table. They are all different from the rest of the elements in their families.
Astatine is radio-active and very rare in nature. Add to this the fact that its most stable isotope has a half-life of only 8.1 hours.