I don't consider the Unknown as legendary Pokemon in the games or the anime. I remember watching the movie that dealed with the mysteries of Unknown, but I just saw them as mystical, not powerful or rare. Any Pokemon can be mystical to an extent, such as Clefairy or Espeon. In the games, they are not rare or powerful either, as I can go catch them whenever I feel like collecting the alphabet. Also, the only move Unknown can learn is Hidden Power, which is a weak, a Normal-Type attack. I guess the fact that they are ancient and varying makes them seem unique, but I feel every Pokemon is unique. Maybe as a group the Unknown can be powerful, but as individual Pokemon, they are not too useful for battling.